Cassin’s sparrow (aimophila cassinii) status assessment and conservation plan Biological Technical Publication BTP-R6002-2000

              i
Janet M. Ruth
Research Ecologist
U.S. Geological Survey
Midcontinent Ecological Sicience Center
4512 Mc Murry Avenue
Fort Collins, CO 80525-3400
970-226-9487
FAX 970-226-9230
janet_ruth@usgs.gov
For additional copies or further information, contact:
Nongame Migratory Bird Coordinator, Region 6
U.S. Fish & Wildlife Service
P.O. Box 25486
Denver Federal Center
Denver, CO 80225
March 2000
Cassin’s Sparrow
(Aimophila cassinii)
Status Assessment
and Conservation Plan
Biological Technical Publication
BTP-R6002-2000
ii U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Recommended citation:
Ruth, J.M. 2000. Cassin’s Sparrow (Aimophila cassinii ) status assessment
and conservation plan. Biological Technical Publication BTP-R6002-1999.
U.S. Department of the Interior, Fish and Wildlife Service, Denver, CO.
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 iii
Table of Contents
TABLE OF CONTENTS TABLE OF CONTENTS ............................................................ iii
LIST OF TABLES AND FIGURES ............................................ vii
EXECUTIVE SUMMARY .......................................................... 1
ACKNOWLEDGMENTS ............................................................ 2
TAXONOMY ............................................................................. 3
LEGAL STATUS........................................................................ 3
United States ................................................................... 3
Mexico ........................................................................... 4
DESCRIPTION .......................................................................... 4
Plumage .......................................................................... 4
Adult ........................................................................ 4
Juvenile ................................................................... 5
NATURAL HISTORY ................................................................ 5
Song ............................................................................... 5
Diet ................................................................................ 6
Territory and Site Fidelity................................................... 6
Breeding Ecology ............................................................. 8
Migratory Behavior ......................................................... 10
RANGE .................................................................................... 10
Breeding ....................................................................... 11
Migration....................................................................... 12
WIntering ...................................................................... 13
Distribution Changes in Response to Precipitation ............... 14
Theory Regarding Unusual “Migration”
Patterns in Cassin’s Sparrows ............................... 17
MONITORING ACTIVITIES ............................................... 18
Breeding Bird Survey (BBS) ....................................... 18
Christmas Bird Count (CBC) ...................................... 19
Breeding Bird Atlases ................................................. 19
POPULATION ESTIMATES AND TRENDS ..................... 19
Population Estimates .................................................. 19
Trends ......................................................................... 19
Possible Explanation of Trends ................................... 22
HABITAT............................................................................... 24
Breeding season habitat requirements ........................ 25
Winter habitat requirements....................................... 27
MANAGEMENT ................................................................... 28
Grazing........................................................................ 28
Burning ....................................................................... 31
Conservation Reserve Program (CRP) ....................... 33
Tamarisk ..................................................................... 33
THREATS.............................................................................. 34
Habitat ........................................................................ 34
Overutilization for Commercial, Recreational,
Scientific, or Educational Purposes ....................... 34
Disease or Predation ................................................... 34
Inadequacy of Existing Regulatory Mechanisms ......... 35
Other natural or ManmadeFactors .............................. 35
Pesticides .............................................................. 35
Population size ..................................................... 35
Nest Parasitism .................................................... 35
iv U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Table of Contents
Suburban Development........................................... 35
Exotic Grasses ........................................................ 36
Grazing .................................................................. 37
Brush Control......................................................... 37
Tamarisk ................................................................ 38
ASSESSMENT AND RECOMMENDATION ............................ 38
Recommendation on current status ................................... 38
CONSERVATION.................................................................... 39
Research Priorities ......................................................... 40
LITERATURE CITED .............................................................. 42
APPENDIX A
STATE SUMMARIES ........................................................... 51
Natural Heritage Program State Ranks Defined ........ 51
UNITED STATES .................................................................. 52
ARIZONA .............................................................................. 52
Summary..................................................................... 52
BBS ............................................................................. 52
CBC ............................................................................ 52
Atlas............................................................................ 52
Research / Monitoring ................................................. 53
Major Populations ....................................................... 55
State Status ................................................................. 55
Natural Heritage Rank ............................................... 55
Habitat Condition........................................................ 55
Threats ........................................................................ 55
Literature Cited .............................................................. 55
NEW MEXICO......................................................................... 58
Summary........................................................................ 58
BBS ............................................................................. 59
CBC ............................................................................. 60
Atlas ............................................................................. 60
Research / Monitoring..................................................... 60
Major Populations ........................................................... 62
State Status ................................................................... 62
Natural Heritage Rank .................................................... 62
Habitat Condition............................................................ 62
Threats ......................................................................... 63
Literature Cited .............................................................. 63
TEXAS 65
Summary....................................................................... 65
BBS ............................................................................. 65
CBC ............................................................................. 66
Atlas ............................................................................. 67
Research / Monitoring..................................................... 67
Major Populations ........................................................... 67
State Status ................................................................... 68
Natural Heritage Rank .................................................... 68
Habitat Condition............................................................ 68
Threats ......................................................................... 68
Literature Cited .............................................................. 69
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 v
Table of Contents
OKLAHOMA ........................................................................... 72
Summary....................................................................... 72
BBS ............................................................................. 73
CBC ............................................................................. 73
Atlas ............................................................................. 73
Research / Monitoring..................................................... 73
Major Populations ........................................................... 73
State Status ................................................................... 73
Natural Heritage Rank .................................................... 73
Habitat Condition............................................................ 73
Threats ......................................................................... 73
Literature Cited .............................................................. 74
COLORADO............................................................................ 75
Summary....................................................................... 75
BBS ............................................................................. 75
CBC ............................................................................. 76
Atlas ............................................................................. 76
Research / Monitoring..................................................... 76
Major Populations ........................................................... 77
State Status ................................................................... 77
Natural Heritage Rank .................................................... 78
Habitat Condition............................................................ 78
Threats ......................................................................... 78
Literature Cited .............................................................. 78
KANSAS ................................................................................. 80
Summary....................................................................... 80
BBS ............................................................................. 80
CBC ............................................................................. 80
Atlas ............................................................................. 80
Research / Monitoring..................................................... 80
Major Populations ........................................................... 81
State Status ................................................................... 81
Natural Heritage Rank .................................................... 81
Habitat Condition............................................................ 81
Threats ......................................................................... 82
Literature Cited .............................................................. 82
WYOMING.............................................................................. 83
Summary....................................................................... 83
BBS ............................................................................. 83
CBC ............................................................................. 83
Atlas ............................................................................. 83
Research / Monitoring..................................................... 83
Major Populations ........................................................... 83
State Status ................................................................... 83
Natural Heritage Rank .................................................... 83
Habitat Condition............................................................ 83
Threats ......................................................................... 83
Literature Cited .............................................................. 84
vi U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Table of Contents
NEBRASKA ............................................................................ 85
Summary....................................................................... 85
BBS ............................................................................. 85
CBC ............................................................................. 85
Atlas ............................................................................. 85
Research / Monitoring..................................................... 85
Major Populations ........................................................... 85
State Status .................................................................... 85
Natural Heritage Rank .................................................... 85
Habitat Condition............................................................ 85
Threats ......................................................................... 85
Literature Cited .............................................................. 86
MEXICO .................................................................................. 87
Summary....................................................................... 87
BBS ............................................................................. 87
CBC ............................................................................. 87
Atlas ............................................................................. 87
Research / Monitoring..................................................... 88
Major Populations ........................................................... 90
State Status ................................................................... 90
Natural Heritage Rank .................................................... 90
Habitat Condition............................................................ 90
Threats ......................................................................... 91
Literature Cited .............................................................. 91
APPENDIX B
HABITAT DESCRIPTIONS................................................. 93
Table B1 ...................................................................... 93
Table B2 ...................................................................... 98
APPENDIX C
LIST OF PRIMARY INDIVIDUALS CONTACTED ........ 101
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 vii
Table of Contents
LIST OF TABLES AND FIGURES
Tables Table 1. Linear measurements (mm) of Cassin’s Sparrow ................ 4
Table 2. Information about nest dimensions,
nest placement, and clutch size ............................................ 9
Table 3. Trends for Cassin’s Sparrow from
Breeding Bird Survey data ................................................ 20
Table 4. Average characteristics of plots
occupied by Cassin’s Sparrows .......................................... 25
Table 5. Densities of singing males per square km
in two habitat types ........................................................... 26
Table 6a. Results of three related studies
comparing Cassin’s Sparrow responses
to grazing in southeastern Arizona .................................... 30
Table 6b. Results of two related studies
describing effects of grazing on vegetative
cover in southeastern Arizona ........................................... 30
Table 7. Total numbers of birds recorded cumulatively
on plots in burned and unburned native and
exotic grassland in southeastern Arizona .......................... 32
Table 8.. Sample size and means for vegetation
associated with wintering grassland bird species
on the Mexican Plateau of Chihuahua, Mexico. ........................ 88
Table 9. Sample size and means for vegetation variables and
habitat associations of wintering grassland bird
species on the Mexican Plateau of Chihuahua, Mexico. ............. 89
Table 10. Average percentage of basal cover and
shrub density in plots where breeding
Cassin’s Sparrows were present and absent ............................ 90
Figures Fig. 1. Breeding and wintering distribution
of Cassin’s Sparrow ............................................................. 10
Fig. 2. Summer distribution map for Cassin’s Sparrow
from Breeding Bird Survey................................................. 12
Fig. 3. Winter distribution map for
Cassin’s Sparrow from Christmas Bird Count ..................... 12
Fig. 4a. Annual indices of abundance by state for
Cassin’s Sparrow from Breeding Bird Survey..................... 15
Fig. 4b. Annual indices of abundance by physiographic strata
for Cassin’s Sparrow from Breeding Bird Survey................ 16
Fig. 5. Annual population trend estimates for
Cassin’s Sparrow from Breeding Bird Survey..................... 21
Fig. 6. Map of Texas showing Edwards Plateau and
South Texas Brushlands ...................................................... 23
Fig. 7. Cassin’s Sparrow distribution in Arizona .................................... 53
Fig. 8. Historical records of Cassin’s Sparrows in New Mexico ............. 59
Fig. 9. Cassin’s Sparrow distribution in Texas ...................................... 66
Fig. 10. Cassin’s Sparrow distribution in Oklahoma ................................. 74
Fig. 11. Cassin’s Sparrow distribution in Colorado .................................. 77
Fig. 12. Cassin’s Sparrow distribution in Kansas .................................... 81
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 1
EXECUTIVE Cassin’s Sparrow (Aimophila cassinii) is a grassland species endemic
SUMMARY to the southwestern U.S. and northern Mexico. Its behavior and ecology have
been shaped by these arid ecosystems and the health
of its populations is dependent on the availability of grasslands that
contain a shrub component. Populations of many grassland birds have
experienced dramatic declines due to the loss and deterioration of
grassland habitats, and there has been concern about the trends in
Cassin’s Sparrow populations. Without better information about
population trends, ecology, and effects of management activities, and
without a stronger emphasis on grassland management, Cassin’s
Sparrows and other grassland species may continue to experience
declines.
Little information exists about historic Cassin’s Sparrow population
levels, and current population estimates for states in the breeding
range are unknown. Significant survey-wide declines are documented
by the Breeding Bird Survey (BBS) between 1966 and 1996. However,
closer examination of these data suggest that the survey-wide trends
are driven by population declines in the Edwards Plateau and South
Texas Brushlands in Texas. No other areas within the core of its range
or on the periphery show consistent, significant trends. In fact, there is
consensus that in the core of its range in New Mexico Cassin’s Sparrow
remains the most abundant breeding bird in grasslands with a shrub
component. Understanding of population patterns in this species is
complicated by large yearly fluctuations in distribution and numbers at
any particular location, apparently in response to precipitation. This is
particularly obvious in the periphery of its range. These dramatic
fluctuations have hampered a clear assessment of population status,
and our understanding has been confounded by the fact that the timing
of BBS data collection does not always coincide with the peak breeding
season of this species. The lack of information about the Cassin’s
Sparrow’s status, ecology, and response to management activities, and
the apparent declines indicated by BBS data, have been cause for
concern.
Habitat disturbance and degradation, mainly due to grazing and
rangeland management practices such as shrub control, fire
management, and planting of exotics, pose threats to Cassin’s Sparrow
populations, as does habitat conversion through suburban development
and agriculture. There is evidence that heavy grazing negatively
affects populations, but the information is primarily available from the
southwestern portion of its range (Arizona) and may not be applicable
throughout the Cassin’s Sparrow’s entire range. Any management
practice that results in complete removal of the shrub component, or
loss of grass cover and an increase in shrub density beyond a threshold
preferred by the species, also poses a threat. Lack of standardized or
coordinated studies of Cassin’s Sparrow ecology and response to
management practices in different geographic portions of its range has
made it difficult to assess its status and make specific management
recommendations. Limited data are available from the wintering
grounds, and from its range (both breeding and wintering) in Mexico.
2 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
ExecutiveSummary
The greatest needs are for determining of the causes of significant declines
where they occur, determining of the effects of various management activities
on Cassin’s Sparrow throughout its range, improved assessments of population
and trends, and a better understanding of the annual population and distribution
dynamics of this species, which shows such dramatic annual distributional
fluctuations.
ACKNOWLEDGMENTS Special thanks are extended to over 50 people from State agencies, U.S. Fish
and Wildlife Service (USFWS), Bureau of Land Management, U.S. Geological
Survey (USGS), U.S. Army, U.S. Forest Service, universities, and private
organizations in the states throughout the range of the Cassin’s Sparrow who
provided information, without which this report could not have been completed.
I particularly want to acknowledge the review and input provided by: Bruce
Peterjohn, USGS Patuxent Wildlife Research Center; Ray Meyer, La Tierra
Environmental Consulting; Brian Locke, Fort Bliss; Stephanie Jones, USFWS;
Tom Shane; John Dunning, Purdue University; Brent Ortego, Texas Parks and
Wildlife; and Chuck Hunter, USFWS. Front cover illustration by Janet M.
Ruth. Joanne Munro assisted with the range map. Stephanie Jones, USFWS
Region 6 Nongame Bird Coordinator, and Bill Howe, USFWS Region 2
Nongame Bird Coordinator, supported this project and made it possible. Layout
and production provided by Beverly Boecher, USFWS Region 6 Visual
Information Specialist.
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 3
TAXONOMY Common Name: Cassin's Sparrow
Spanish Common Names: Gorrion de Cassin or Zacatonero de Cassin
Scientific Name: Aimophila cassinii
Order: Passeriformes
Family: Emberizidae
The first Cassin's Sparrow was described in 1852 by Samuel W.
Woodhouse from a specimen collected near San Antonio, Texas, and
given its species name in honor of John Cassin, a Philadelphia
ornithologist (Terres 1980). The species was originally known as
Zonotrichia cassinii (AOU 1998). It was subsequently and variously
assigned to the genus Peucaea and eventually to Aimophila around the
turn of the century (Wolf 1977). Much of the confusion seems to have
stemmed from a serious lack of knowledge about the anatomy and life
history of the species included in the genus.
There have been several substantial treatments of the taxonomy of
species within the Aimophila genus (Wolf 1977, Storer 1955) and a
comparison of the song patterns of Aimophila sparrows (Borror 1971),
but they have focused primarily on evaluating the evolutionary
development of these species in order to determine whether this genus
actually consists of an unnatural assemblage of species (actually
representing several taxonomic groups or divergent forms) (Storer
1955). None of these publications called into question the placement of
Cassin's Sparrow within this genus in what is called the "botterii
complex" - Botteri's Sparrow (Aimophila botterii), Bachman's Sparrow
(A. aestivalis), and Cassin's Sparrow (A. cassinii).
No subspecies or races of Cassin's Sparrow are recognized (Pyle 1997;
AOU 1957; Dunning et al. 2000).
LEGAL STATUS
United States Cassin's Sparrow is protected under the Migratory Bird Treaty Act of
1918. The Cassin's Sparrow is not listed as threatened or endangered
under the federal Endangered Species Act. It is included on the list of
"Migratory Nongame Birds of Management Concern in the United
States: the 1995 List" issued by the U.S. Fish and Wildlife Service
(USFWS 1995). In that report, it is listed as a species of concern in
USFWS Regions 2 and 6.
Cassin's Sparrow is listed on the joint National Audubon Society-
Partners in Flight "WatchList" (Muehter 1998), a list of species of
national conservation concern. The WatchList identifies human
alteration of habitat and loss of suitable mixed grass-shrub habitat as
threats.
The Nature Conservancy (TNC) global rank for Cassin's Sparrow is
G5, indicating a demonstrably secure population.
Cassin's Sparrow does not have any legal status in any of the states in
which it occurs. Appendix A provides other details of its status in these
states.
Taxonomy
4 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Mexico Cassin's Sparrow is protected under the Convention for the Protection of
Migratory Birds and Game Mammals of 1936, but has no other legal status.
DESCRIPTION The Cassin's Sparrow is a fairly large, plain, grayish sparrow that lacks
conspicuous markings. In flight, the long, roundish tail is obvious and
the white tips of the tail feathers are sometimes apparent. This species
is most easily identified by its distinctive song and dramatic skylarking
behavior during the breeding season. Although often characterized in
the literature as secretive and difficult to observe when not singing
(Williams and LeSassier 1968; Oberholser 1974; Kaufman 1990),
Schnase (1984) observed that Cassin's Sparrows readily accommodated
the presence of an observer, especially early in the breeding season.
Average body mass of males in Kansas in June (n = 11) is 17.8 g (range
= 16.9 - 18.5 g) (Rising 1996); average body mass (both sexes) in
southeastern Arizona year round (n = 125) is 18.3 (± 2.9) g (Dunning
and Bowers 1986). Size is 13-15 cm, with males being slightly larger;
the sexes are similar in coloration (Rising 1996). Table 1 shows linear
measurements from collected specimens.
Table 1. Linear measurements (mm) of Cassin's Sparrow. Data shown as mean (range; n).
From Wolf (1977).
Measurement Males Females
Wing Chord 64.0 (62 - 67; 43) 62.0 (59 - 66; 16)
Tail Length 68.0 (64 - 71; 13) 66.0 (62 - 71; 13)
Tarsus 19.9 (18.5 - 21.4; 45) 19.8 (18.8 - 21.4; 37)
Culmen Length
(from anterior of nostril) 7.7 (6.8 - 8.3) 7.5 (7.0 - 8.2)
Bill Depth 4.9 (4.5 - 5.5) 4.8 (4.5 - 5.1)
Plumage Plumage references (Rising 1996; Byers et al. 1995; Kaufman 1990).
Adult - The head is brown streaked with gray and dark brown; the
supercilium is buff, and there is a thin, dark brown submoustachial
stripe. The bill is brownish gray, with darker upper mandible and pale
bluish gray tomial edge and lower mandible. The iris is dark brown.
The chin, throat and breast are pale gray or brownish gray; the belly is
whitish; and there are a few well-defined dark brown or black streaks
on the lower flanks. On the back, the mantle and scapulars are
described as brown or gray with a rusty tinge, the feathers having dark
brown subterminal spots and edged with buff or gray, giving a scaly or
variegated appearance. Wings are brown; greater coverts are broadly
tipped and narrowly edged with buff or grayish white, forming a wing
bar variously described as fairly conspicuous to indistinct. The alula is
pale yellow. Feathers in the upper tail coverts have a gray edge, a
brown center, and a black subterminal crescent. The undertail coverts
are buffy. Most of the upper side of the tail is dark, dusky brown, but
the central two rectrices are pale brownish gray with a serrated dark
central strip that spreads out into a suggestion of faint crossbars. The
lateral two rectrices are edged and tipped in pale gray or white, with smaller
Legal Status
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 5
pale areas at the tips of the next two pairs inward. This is sometimes noticeable
on a bird flushing or flying away, but it is not always apparent, and by late
summer, pale tips may be partly or completely worn away. Legs are described
as dull pinkish or dark flesh.
Juvenile - Juveniles are similar to adults with a brown back, feathers with buffy
tips and darker brown central streaks, greater coverts edged with white, and
light streaking on breast and throat.
Byers et al. (1995) noted that some birds, mainly in the eastern part of
their range, tend to be more rufous above, slightly buffier below, and
have plainer tails with less obvious shaft streaks and barring on the
central rectrices. Although rarer, even in the eastern part of the range,
the rufous morph has been observed as far away as the Farallon
Islands off California (J. Dunning pers. commun.).
Willoughby (1986) reports on an unusual sequence of molts and
plumages in Cassin's and Bachman's Sparrows – replacement of all
pennaceous body plumage twice within a bird's first six months of age,
and a gradual molt of body feathers in adults throughout the breeding
season. Designated as a presupplemental molt, this molt has been fully
documented in certain species only recently, having been found in 16
species of North American passerines to date (Pyle 1997).
NATURAL HISTORY
Song The Cassin's Sparrow's primary song consists of six note complexes,
beginning with a soft double or single introductory note, followed by a
long, high musical trill on one pitch, and (usually) two lower, well-spaced
musical notes, all with a slight minor-key quality. There is
enough individual variation in this song that it has been used as a
means of identifying individual males in population studies (Schnase
and Maxwell 1989). A secondary song, or "chitter" song (Wolf 1977),
consists of a series of chips, trills, and buzzy notes preceding the
primary song (Schnase 1984). Cassin's Sparrows also give a variety of
chitter calls and chip notes that have been assigned various roles by
different authors, including pair bond maintenance, communication
with fledglings, alarm calls, territory defense, etc. (Kaufman 1990;
Schnase 1984; Wolf 1977). Unusual conditions may induce this species
to sing at unusual times of year (Kaufman 1990).
Territorial males sit in low bushes or grass, or on the ground to sing,
but often give spectacular flight-songs. At the beginning of the
breeding season, all song is from a stationary, exposed perch and often
involves reciprocal proclamation of the primary song among males.
Flight songs and skylarking are infrequent until later, in association
with the presence of returning females (Schnase et al. 1991; Schnase
1984). In flight songs (or skylarking), the territorial male flies up from
an exposed perch, such as a bush, to as much as 5 - 10 m in the air, then
sings as he glides or flutters down in an arc to a nearby bush or the
ground. During the descent, wings are held flat, the head is arched
backwards, and the tail is elevated. Song can be heard from mid-February to
early September, depending on location, with considerable night singing at the
Discription
6 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
height of the season reported by some (Rising 1996; Howell and Webb 1995;
Schnase 1984; Oberholser 1974).
Diet The summer diet of Cassin's Sparrows consists primarily of insects,
especially grasshoppers, caterpillars, and beetles. Additional insects
specifically mentioned in the literature include true bugs, ants, bees,
wasps, weevils, spiders, snails, and moths (Dunning et al. 2000;
Kaufman 1996; Bock et al. 1992; Oberholser 1974; Williams and
LeSassier 1968). The young are fed almost entirely insects (Kaufman
1996). Bock et al. (1992) note that observations of a Cassin's Sparrow
nest for 18 hours in 1984 showed that of 208 insects delivered to
nestlings, 197 (95%) were acridid grasshoppers. However, Wolf (1977)
reported that the stomachs of ten adults taken during the breeding
season (late June and early July) contained animal and vegetable
matter in about equal proportions (52% and 48%, respectively; range =
5 - 95%). He also found that five migrant Cassin's Sparrow stomachs
contained 99% animal material (range = 90 - 100%). There is a report of
Cassin's Sparrows eating flower buds of blackthorn bush (Condalia
spathulata) in season (Oberholser 1974). In fall and winter, Cassin's
Sparrows eat the seeds of weeds and grasses (Kaufman 1996; Williams
and LeSassier 1968). Oberholser (1974) particularly mentions the
consumption of seeds of chickweed (Alsinaceae family), plantain
(Plantago spp.), woodsorrel (Xanthoxalis spp.), sedge (Carex spp.),
panicum (Panicum spp.), other grasses, and sorghum (Sorghum spp.).
Schnase (1984) reports observing birds drinking water from a small
pool immediately following a rain. Although Williams and LeSassier
(1968) report that Cassin's Sparrows seem to exist very well without
drinking water, their conclusion appears to be based on the limited
number of recorded observations of this species drinking water, the
distance of most nesting areas from water, and the fact that birds rarely
leave their territories.
Cassin's Sparrows forage mostly or entirely on the ground, hopping
about in relatively open areas, taking items from the ground or from
plant stems (Kaufman 1996, Schnase 1984). When flushed, they fly to a
bush or fence, or may drop back into the grass (Rising 1996). Schnase
(1984) reported that foraging occurred in a slow, methodical manner.
Foliage gleaning from within mesquite (Prosopis spp.) and other
shrubs was only prominent after nestlings and fledglings were present.
Fledglings apparently acquired most of their food in this manner rather
than on the ground.
Territory and Cassin's Sparrows establish and maintain breeding territories solely
Site Fidelity by song (Schnase et al. 1991); mating, nesting, and feeding occur within
these territories. Mean territory size over two years (n = 21) in south-central
Texas was found to be 2.6 (± 0.5) ha (Schnase 1984); the
information was also presented as a mean density of 11 birds per 40.4
ha. A number of other studies have reported Cassin's Sparrow
breeding densities. However, it is unlikely that these numbers can be
compared with each other since various methods of data-gathering and
analysis were used. Nevertheless, they do provide some idea of the
Natural History
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 7
possible densities for this species. Another study in south-central
Texas found densities ranging from 33 birds per 40.4 ha in scrubby
mesquite grassland to six birds per 40.4 ha in bottomland mesquite
woodland (Maxwell 1979). A Breeding Bird Census on a 61 ha plot on
the Buenos Aires National Wildlife Refuge in Arizona found 15 Cassin's
Sparrow territories for a density of ten territories per 40 ha (Gordon
and Leitner 1996). By comparison, A. Flesch (written commun.)
reports that in a good year, Buenos Aires NWR supported densities
of 86 singing males per 40 ha. In an unpublished study of bird
communities in sacaton (Sporobolus sp.)-mesquite grasslands along the
San Pedro River in Arizona, one transect showed annual peak breeding
densities (based on Emlen transects) of 41, 66, 91, and 19 birds/40 ha in
respective years; a second transect showed annual peak densities of 39,
26, and 14 birds/40 ha (D. Krueper written commun.). A study of avian
communities within a variety of habitats along the Pecos River
(Hildebrandt and Ohmart 1982) found a range of Cassin's Sparrow
densities (presenting calculations from the summer periods of June -
July and August - September). They found 6 - 12 birds per 40 ha in
sparse (little foliage volume above 1.5 m) honey mesquite (Prosopis
glandulosa) habitat; 6 - 12 birds per 40 ha in sparse four-winged
saltbush (Atriplex canescens) habitat; 6 - 24 birds per 40 ha in low (little
foliage above 3 m) to sparse tamarisk (Tamarix chinensis) habitat; 2 - 7
birds per 40 ha in cleared communities dominated by annual and
perennial weeds and shrubs; and 3 - 4 birds per 40 ha in cleared
communities dominated by grass.
Schnase (1984) found no overlap in territorial boundaries, and adjacent
territories were separated by 15-75 m; Johnsgard (1979) reported that
territorial males may be spaced 50-100 yards apart. Both Williams and
LeSassier (1968) and Johnsgard (1979) report observations of clusters
or groups of breeding Cassin's Sparrows and suggested that they may
be semi-colonial.
Research on site fidelity in wintering grassland birds in southeastern
Arizona (Gordon In Press a and b) found high within-season recapture
rates for Cassin's Sparrows on seven ha plots, indicating very
sedentary behavior for this species. Using three years of data, he
calculated the recapture event rate - rer (the number of recaptures
divided by the number of opportunities for recapture). Cassin's
Sparrow (rer = 0.11) and Grasshopper Sparrow (Ammodramus
savannarum) (rer = 0.09) remained within a very small area (or
territory) during the winter, showing significantly higher (P < 0.05)
recapture rates than Baird's Sparrow (A. bairdii) (rer = 0.04),
Savannah Sparrow (Passerculus sandwichensis) (rer = 0.01), Vesper
Sparrow (Pooecetes gramineus) (rer = 0.02), and Brewer's Sparrow
(Spizella breweri) (rer = 0.004). Patterns of between-year capture and
recapture rates in wintering Cassin's Sparrows indicate great
variability, possibly tied to between-year nomadism, between-year
population fluctuation, or expansion-retraction along the edge of their
range (C. Gordon written commun.). For example, at the same plots
with the same level of effort, nine Cassin's Sparrows were captured in the
winter (January to March) of 1997 and 43 were captured in 1998. One of the
Natural History
8 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
nine individuals from 1997 was recaptured in 1998, and in 1999, one "1997 bird"
(a different one), and one "1998 bird" were recaptured (all of these recaptures
on the same seven ha plots on which they were first banded), indicating that
between-year site fidelity does exist to some extent.
Breeding Ecology Breeding in Cassin's Sparrow across its range can occur from March
through September. Rising (1996) notes that breeding appears to take
place in early summer in coastal Texas, Kansas, Colorado, and north-east
New Mexico, and in late summer (after the rains) in southeastern
Arizona. See Appendix A for specific information about breeding
timing and records from individual states.
Schnase (1984) reports indirect evidence that Cassin's Sparrows form
stable, monogamous pair bonds. Some sources believe that this species
may double-brood (Baicich and Harrison 1997; Rising 1996; Wolf 1977).
However, Schnase (1984) reported that, although renesting took place
in three cases where abandonment or destruction of a clutch occurred
before mid-June, there was no evidence that any of the 25 pairs he
observed successfully reared more than one clutch.
Schnase (1984) reports that females returned later than males in the
spring and their return coincided with the onset of skylarking by males.
Males pursued females to the border of their territories. Males
frequently pursued females in slow, horizontal flight no more than 2-3
m above the ground while producing the "chitter" call. Copulation has
only been observed rarely. Schnase (1984) reports that the male
initiates a courtship display involving an erect head and tail with wings
fluttering in an outstretched position which is followed by copulation;
this was seen twice on the ground and once in a low mesquite.
Cassin's Sparrows nest on or near the ground. There are
approximately equal numbers of reports of ground nests and nests
elevated a few inches off the ground in shrubs or other vegetation
approximating shrub structure (e.g., cactus). Nests on the ground are
usually concealed in tall grass or grass tufts, or at the base of shrubs or
Opuntia cacti (Baicich and Harrison 1997; Rising 1996; Schnase 1984,
Johnsgard 1979; Williams and LeSassier 1968). Nests in shrubs are
rarely located more than 12 inches above the ground (Table 2). They
are often found in the midst of a tangled patch of Opuntia cactus
(Williams and LeSassier 1968). The nest is a deep cup constructed of
dead grasses, weed stems, bark and plant fibers, and sometimes grass
flowers. It is lined with finer grasses and grass flowers, rootlets, and
sometimes hair (Baicich and Harrison 1997; Rising 1996; Williams and
LeSassier 1968).
Natural History
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 9
Table 2. Information about nest dimensions, nest placement, and clutch size from studies in
southeastern Arizona (Maurer et al.1989), Texas (Schnase 1984; Schnase et al. 1991)
and New Mexico (A. Pidgeon written commun.).
Maurer et al. 1989 Schnase 1984; Pidgeon written
Schnase et al. 1991 commun.
Avg. nest height
from ground (cm) 10.8 (±5.9) (n = 18) 4.0 (±2.3) (n = 10) 15.0 (n = 6)
Avg. cup depth (cm) 5.4 (± 0.4) (n = 17) 6.4 (± 1.0) (n = 10) N/A
Avg. cup width (cm) 6.5 (± 0.4) (n = 18) 5.9 (± 0.6) (n = 10) N/A
Nest plant height (m) 0.7 (± 0.2) (n = 18) 0.4 (±0.1) (n = 9) 0.6 (n = 6)
Avg. clutch size 3.0 (±0.9) (n = 10) 2.2 (±?.?) (n = ??) 3.7 (n = 10)
The subelliptical eggs of Cassin's Sparrows are white and unmarked,
smooth and slightly glossy. The average size is 19 x15 mm. Cassin's
Sparrows lay three to five eggs, usually four (Table 2) (Baicich and
Harrison 1997; Johnsgard 1979; Williams and LeSassier 1968).
Berthelesen and Smith (1995) found a slightly larger average clutch size
in their study (4.4 ± 0.61, n = 34).
Little documented evidence was found on the incubation period for
Cassin's Sparrow. Baicich and Harrison (1997) estimate it at ten days.
Schnase et al. (1991) observed incubation of 11 days in one nest. They
found that females laid one egg each morning and began incubation
with the third egg of a four-egg clutch. Males were never observed at
the nest, and Schnase et al. (1991) assumed that females were the
primary brooders. Dunning et al. 2000 Data for three Arizona nests in
which the incubation period was 11 days, 11 days, and 9 days for the
three nests (Dunning et al. 2000).
Nestlings are altricial, with sparse, very dark down (Baicich and
Harrison 1997); Schnase (1984) described nestlings as naked except for
sparse, light-gray down on the head and back, with pronounced yellow
rictal flanges and dark red mouth lining. Most records indicate that the
nestlings are tended by both parents (Baicich and Harrison 1997;
Williams and LeSassier 1968; Johnson 1956), but Schnase et al. (1991)
found little evidence regarding male-female cooperation in feeding
nestlings. The parents are extremely secretive in their approach to the
nest, making nest finding very difficult. A. Flesch (written commun.)
reported observing parents carrying fecal sacs. Nestlings leave the
nest at approximately nine days (Baicich and Harrison 1997, Schnase et
al. 1991). Schnase et al. (1991) did find that females assumed primary
responsibility for care of the young once they fledged. In addition, they
found that within two days of fledging, young were capable of repeated
flights of 10-15 m, and that periods of independent foraging in
vegetation and on the ground were common for fledglings eight days
after leaving the nest.
Eventually fledglings became less dependent and associated with
fledglings from adjacent territories in flocks of as many as 10-20
individuals that moved throughout territories (Schnase 1984).
Natural History
10 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Very little information is available regarding productivity in Cassin's Sparrows
and the minimal information available from different studies is widely variable.
Schnase (1984) observed a total of 40 fledglings produced by 25 males for a 1.6
fledglings/male ratio over three summers. Two of the 25 males were unpaired,
and five of 23 pairs were not successful in fledging any young. Schnase
identified productivity by male because individual birds in the study (males)
were identified primarily by variation in song. The presentation of these data
made further interpretation difficult. A study of breeding bird use of
Conservation Reserve Program (CRP) lands in the Texas Panhandle
(Berthelsen and Smith 1995) estimated Cassin's Sparrow nest success
(n = 34) as 44 (± 0.89) % through fledging, using the Mayfield method.
Preliminary data from a study on Fort Bliss (n = 10) found the mean
number fledged per nest was 1.9 (A. Pidgeon written commun.).
Migratory Behavior Nothing is known about the migratory behavior of this species.
RANGE Cassin's Sparrow is a species of the southwestern U.S. and central Mexico
(Figure 1).
Figure 1. Breeding and wintering distribution of Cassin’s Sparrow. They are found in suitable
grassland/shrubland habitat within the boundaries shown.
Natural History
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 11
There is little information about the historical range of Cassin's Sparrow, and
therefore little evidence of whether the species' range has expanded or
contracted. This is complicated by the annual fluctuations in distribution of this
species. However, records in the last twenty years of Cassin's Sparrows
breeding in Wyoming and Nebraska may represent an expansion of their
distribution, in some years at least. Hubbard (1977) suggests that the more
recent documentation of breeding Cassin's in southwestern New Mexico and
southeastern Arizona may also represent an expansion of their breeding range.
Breeding Even the current breeding range for this cryptic species continues to
be difficult to define. Cassin's Sparrows are sometimes very common
but are irregular, with large numbers often appearing in an area after
good rains have caused vegetation to turn green (Kaufman 1996). In
addition, because there have been so few records of Cassin's Sparrow
nests, many of the descriptions of breeding range are based on the
presence of singing males.
The breeding distribution of the Cassin's Sparrow is described by
Breeding Bird Survey (BBS) data (Figure 2).
Figure 2. Summer distribution map for Cassin’s Sparrow from Breeding Bird Survey (BB) 1982-1996
(Sauer et al. 1997). This is the average relative abundance of the species detected per BBS
route per year.
BBS SUMMER DISTRIBUTION MAP 1982-1996
Range
12 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Figure 3. Winter distribution map for Cassin’s Sparrow from Christmas Bird Count (CBC) 1966 - 1989
(Sauer et al. 1996). This is average relative abundance of the species per CBC circle.
CBC WINTER DISTRIBUTION 1966 - 1989
A compilation of existing descriptions of the species' breeding
distribution from the literature yields the following (Howell and Webb
1995; Rising 1996; AOU 1998): Cassin's Sparrows breed from
southwestern Nebraska, western Kansas, southeastern Colorado
(irregularly into northeastern Colorado), southern and eastern New
Mexico, western Oklahoma, western two-thirds of Texas, Chihuahua,
Coahuila, in the interior south to Zacatecas and San Luis Potosí, and on
the Atlantic slope from Nuevo Leon to Tamaulipas. Sporadic records
exist for central and eastern Wyoming, and southwestern South
Dakota. See Appendix A for detailed information on distribution within
individual states.
Migration Cassin's Sparrows are migratory in the northern part of their range,
withdrawing basically into the southern part of their range and
possibly a little farther south in Mexico for the winter. Defining the
arrival and departure times for Cassin's Sparrows throughout their
range is somewhat difficult because the data are derived from
miscellaneous arrival, breeding, and nesting dates. A migratory flux of Cassin's
Sparrows arrives in Texas in March, even in areas where some individuals
overwinter (Hubbard 1977), and Texas has the earliest breeding records for
Cassin's Sparrow - early March (Hubbard 1977; Oberholser 1974). They are
Range
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 13
reported breeding in Mexico in April (Howell and Webb 1995). They also
return to New Mexico in large numbers in early April (Hubbard 1977) and to
Oklahoma by late April (Baumgartner and Baumgartner 1992). The first nest-building
in southeastern Colorado was observed in mid-May (J. Bradley pers.
commun.), but the Colorado Breeding Bird Atlas records most nesting activity
beginning in early June (Kingery 1998). Egg dates for Kansas begin in mid-
May (Johnsgard 1979). Breeding of Cassin's Sparrows begins in southeastern
Arizona in July (Maurer et al. 1989; Monson and Phillips 1981).
It appears that Cassin's Sparrows may leave Colorado and Oklahoma
by the end of August. The last nest monitored for fledglings in
southeastern Colorado during a 1996 study was in the first week of
August (J. Bradley pers. commun.), and Cassin's Sparrows are
recorded through August in Oklahoma (Baumgartner and
Baumgartner 1992). The majority leave New Mexico by late
September (Hubbard 1977). In Texas, breeding records occur through
early August and the winter season for Cassin's Sparrows begins in
mid-October (Oberholser 1974). Breeding in southeastern Arizona and
Mexico continues through September (Maurer et al. 1989; Monson and
Phillips 1981; Howell and Webb 1995). Migration patterns in Arizona
continue to be poorly defined, and although Cassin's Sparrows winter
over most of their breeding range in Mexico, there may be some
withdrawal south during that period (Howell and Webb 1995).
Cassin's Sparrows sometimes turn up far outside their normal range
during migration, with scattered records from coast to coast (Kaufman
1996). Roberson (1980) reports five records of spring and fall vagrants
in the Southeast Farallon Islands, California, as well as spring and
summer nomads in potential breeding habitat in southern California
(25 records), where singing males defended territories for a brief time
and then disappeared. They have been reported as casual or accidental
in Nevada, Ontario, Nova Scotia, Missouri, Illinois, Indiana, and New
Jersey (Rising 1996; AOU Checklist 1998; Dunning et al. 2000). Russell
and Monson (1998) also documented unexpected locations for Cassin's
Sparrows outside their normal range in Mexico in years of above-normal
rainfall (e.g., a dozen singing Cassin's Sparrows among sand
dunes near Puerto Peñasco (far northwestern Sonora) in late March-early
April 1984.
There are reports of gender differences in migration patterns, with
males appearing in Texas in the spring at least two weeks before the
females (Schnase et al. 1991; Schnase 1984).
Wintering The winter distribution of Cassin's Sparrows in the U.S. is based on
Christmas Bird Counts (Figure 3).
Unfortunately, similar information does not exist for its primary winter
range in Mexico. A compilation of the descriptions in the literature of
the species' wintering range results in the following (Howell and Webb 1995;
Rising 1996; AOU 1998): Cassin's Sparrows winter in southeastern Arizona,
only rarely or sporadically in southern New Mexico, in western and south-central
Texas, and into Mexico on the Pacific slope from Sonora through
Range
14 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Sinaloa to Nayarit and south in the interior including Chihuahua, Coahuila,
Zacatecas, San Luis Potosí and Guanajuato. See Appendix A for available
details on winter distribution in individual states.
Distribution Changes There is a consensus, throughout the literature and among the
In Response To contacts made for this report, that breeding Cassin's Sparrows are
Precipitation very responsive to precipitation. However, little is offered in the way
of evidence or specific descriptions of that response. In addition, there
appear to be some differences of opinion regarding how this response
manifests itself. Many references associate variations in Cassin's
Sparrow abundance with variations in precipitation patterns and
associated vegetation growth, but some suggest that there are more
Cassin's Sparrows when there is more rain (Phillips 1944; Williams and
LeSassier 1968), while others suggest that there are more birds when
there is less rain (M. Howery pers. commun.; C. Sexton, written
commun.; Andrews and Righter 1992). These differences may be
related to different geographic locations and associated weather
patterns. Baumgartner and Baumgartner (1992) spoke most clearly
about this. They noted that Cassin's Sparrows are most abundant in
western Oklahoma, but that they expand eastward in the state during
periods of drought, when vegetation is stunted and pastures that
normally sustain tall or mixed grasses approximate the more arid
conditions of western plains.
Maurer (1985) conducted a broad study of avian community responses
to temporal distribution of rainfall and spatial distribution of mesquite
trees on the Santa Rita Experimental Range (SRER). Although the
focus was on community-level responses, he did present information
specific to Cassin's Sparrows. In southeastern Arizona, July through
September is the season of greatest rainfall, with an associated high
grass productivity (Cable 1975) and increase in available insect biomass
(Maurer 1985). During the first year of this study, the site received
average rainfall; during the second year it received abnormally high
winter precipitation and somewhat delayed, but normal, summer
rainfall. The following data were extracted from Maurer (1985), but did
not include statistical tests for significance. In grassland habitats with
low mesquite densities (the habitat preferred by Cassin's Sparrows),
Cassin's Sparrow densities increased in the breeding season following
greater precipitation (1982 - 43.2 males/km2; 1983 - 71.2 males/km2).
Even in mesquite savannah habitat (less preferred by Cassin's
Sparrows), bird densities increased following the year of greater
precipitation (1982 - 4.9 males/km2; 1983 - 34.5 males/km2).
To summarize these observations, it appears that the broad temporal
fluctuations in Cassin's Sparrow distribution and numbers (Figure 4a
and 4b) are a response to changes in timing and amount of precipitation.
Range
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 15
Figure 4a. Annual indices of abundance by state for Cassin’s Sparrow from Breeding Bird Survey.
Annual indices of abundance are estimated as residuals from the route-regression. The
line, depicting the predicted trend in counts over time, is drawn using the regional
trend estimate and a regional average count (Sauer et al. 1997).
Range
16 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Figure 4 b. Annual indices of abundance by physiographic strata for Cassin’s Sparrow from
Breeding Bird Survey. Annual indices of abundance are estimated as residuals from the
route-regression. The line, depicting the predicted trend in counts over time, is drawn
using the regional trend estimate and a regional average count (Sauer et al. 1997).
Range
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 17
The most likely factors to which they are ultimately responding are changes in
vegetative structure and vigor. Thus, at the eastern and northern edges of their
range, where there is usually more rainfall and the grass structure is usually too
tall or dense for Cassin's Sparrow habitat, Cassin's Sparrows only expand or
increase in abundance in dry years. In these dry years the vegetative structure
is stunted and sparse, and more closely approximates their preferred habitat. In
contrast, at the southwestern edge of their range, where rainfall can be quite
sporadic and the grass structure in dry years is not sufficient to support Cassin's
Sparrows, Cassin's Sparrows only expand or increase in abundance in wet
years, when the vegetative structure is lush enough for them. In the core of
their range, a similar pattern may be occurring, which is manifest as increases in
abundance in years with precipitation and habitat structure that are optimal, and
decreases in suboptimal years. However, such fluctuations would be more
difficult to observe because they are not accompanied by expansions in
distribution.
There remains much to understand about the factors affecting Cassin's
Sparrows dramatic fluctuations in distribution. Although they appear
to be responding to the effects of changing precipitation patterns, the
actual process by which precipitation affects them is unclear. They may
be responding to changes in vegetative structure or composition
needed for nesting, hiding from predators, or courtship displays, as
suggested above. In that case, the rapid response noted in the birds,
which often seem to appear and begin singing within days of the first
rains of the season and before the vegetation has responded, remains to
be explained. It may mean that they have evolved to respond to the
initiation of rain itself as a proximal cue, rather than the actual
greening of vegetation which follows. An alternative explanation for
the Cassin's Sparrow response to precipitation is that they are
responding to changes in prey populations (particularly insects such as
grasshoppers) that in turn are responding to changes in precipitation.
There is no direct evidence of this association, but Maurer (1985)
concluded that the influence of rainfall on insect productivity strongly
affected the avian community in Arizona, and there is evidence that
grasshopper populations respond positively to precipitation in the
southwestern U.S. (Capinera and Horton 1989; Capinera and
Thompson 1987; Nerney 1961).
Theory Regarding There remains an unresolved controversy regarding the migration
Unusual “Migration” and breeding patterns of Cassin’s Sparrows. Phillips (1944) was the
Patterns In first to specifically address this issue. He based his hypothesis on
Cassin’s Sparrows the lack of documentation of breeding in Arizona at the time, and his
observations that large numbers of Cassin's Sparrows did not appear in
the state until July and August, when males sang and were in breeding
condition (enlarged testes). Phillips concluded that the species did not
breed in Arizona but rather migrated east-to-west, from its principal
breeding areas in Texas to southeastern Arizona, where it was an
abundant fall transient and an irregular, rather common winter
resident.
Range
18 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Subsequently, nesting Cassin's Sparrows have been documented in southeastern
Arizona in late summer (Monson and Phillips 1981; Ohmart 1966). In response
to this evidence, Ohmart (1969) suggested that a single population of Cassin's
Sparrows took advantage of the food source provided by spring rains on the
Great Plains, and then moved southwest for a second breeding season in the
Sonoran Desert in response to food sources produced there by late summer
rains. Wolf (1977) suggested that the late summer males might be individuals
that did not mate during the breeding season (presumably on the southern Great
Plains) and were migrating early without undergoing gonadal regression.
Hubbard (1977) espoused the view that Cassin's Sparrows may have bred very
locally and perhaps irregularly in Arizona for some time, but that the bulk of the
late summer birds were nonbreeders.
Although there are no published studies addressing this issue, there
are very strong opinions about the hypothesis. There are those who
believe that Cassin's Sparrows are also present in southeastern
Arizona during spring and early summer but are so cryptic and
secretive that they are simply not observable until they begin singing
and skylarking in response to late summer rains. Others are very
insistent that if Cassin's Sparrows were present, even if acting
secretive, experienced birders and researchers would have detected
them. Still others are more comfortable with an intermediate
interpretation – that Cassin's Sparrows are simply very opportunistic
and move around on a much larger scale than most species, looking for
the right combination of rain, vegetation, and food resources needed for
breeding. In any case, this is an unresolved question regarding
Cassin's Sparrow ecology.
MONITORING ACTIVITIES
Breeding Bird Survey Monitoring of breeding Cassin's Sparrows on a range-wide basis in
(BBS) the United States, and to a minimal extent in Mexico, is restricted to
the BBS. The interpretation of BBS data is limited in at least some
areas by the low numbers and distribution of routes completed in an
area, by the highly variable nature of Cassin's Sparrow annual
distributions, and by the fact that their most active, observable
breeding season at times coincides with the early summer timing of
BBS data collection and at times does not.
A three-year BBS feasibility study was conducted in Mexico from 1993
- 1996. None of those routes have been surveyed subsequently. With
the exception of a few routes that are sporadically surveyed by U.S.
citizens, the BBS does not exist in Mexico at this time. Expansion of the
BBS program, or an equivalent, into Mexico would provide valuable
information about Cassin's Sparrow populations there.
Results of BBS data analysis and additional details about BBS data
from individual states and Mexico are presented elsewhere (Trends
section and Appendix A).
Range
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 19
Christmas Bird Count The Christmas Bird Counts in the southwestern part of the United
(CBC) States only cover a small portion of the Cassin's Sparrow winter range (Root
1988). There are a few CBCs done in Mexico by coalitions of U.S., Mexican,
and in some cases Canadian citizens, primarily in Sonora, Chihuahua, and
Tamaulipas, although there is no organized program for the country.
Breeding Bird Atlases Breeding Bird Atlases (BBAs) are conducted by states to document the
distribution and reproductive status of breeding birds within their
borders. Most BBAs are five-year projects, however, some states plan
to periodically repeat Atlas projects. Once repeated, atlas projects will
provide valuable information on changes in Cassin's Sparrow
distribution. Although only one state within its range (Colorado) has
published its breeding bird atlas, Cassin's Sparrows were detected
during BBA data collection in six states - Arizona (Arizona Breeding
Bird Atlas unpub. data), Texas (Arnold and Benson unpub. data),
Oklahoma (Oklahoma Breeding Bird Atlas unpub. data), Kansas (Busby
and Zimmerman unpub. data), Colorado (Kingery 1998), and Nebraska
(Sharpe et al. unpub. data). New Mexico and Wyoming have not
conducted Breeding Bird Atlas projects. Summaries of BBA data for
each state can be found in Appendix A.
Although there are no other range-wide monitoring programs, there
are some site-specific monitoring projects being conducted at various
locations (Appendix A).
POPULATION Great fluctuations in annual numbers at any particular location make
ESTIMATES AND TRENDS it difficult to estimate the overall population of Cassin's Sparrows or monitor
trends in that population.
Population Estimates No population estimates for Cassin's Sparrows were found in the
literature or during the compilation of information for this report.
Trends The best available description of population trends for Cassin's
Sparrow is from BBS data analysis. For purposes of this document,
trends with a P-value < 0.20 are presented; any pattern with a larger P-value
is considered nonsignificant.
Survey-wide data for Cassin's Sparrow during the entire time period
for which BBS data are available (1966 - 1996), show a highly significant
decline (Table 3). During this same time period, Cassin's Sparrow data
in Texas show a similar, highly significant downward trend, with
Colorado and Kansas also showing a significant decline. No other state
data within the bird's range show any significant trends in either
direction for the entire BBS time period (Sauer et al. 1997). Figure 4a
shows annual indices of abundance for New Mexico, Texas, Colorado,
and Oklahoma. Figure 5 maps the variation in population trend
estimates for this species over the entire time period covered by BBS.
Monitoring Activities
20 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Table 3. Trends (average percent change per year) for Cassin's Sparrow from Breeding Bird
Survey data from 1966-1996 (Sauer et al. 1997; Sauer unpub. data a).
P-value: * 0.10 < P < 0.20 ** 0.05 < P < 0.10; *** 0.01 < P < 0.05; **** P <0.01(1)
Total Time Period
1966 - 1996 1966 - 1979 1980-1996
Trend p - value n Trend p - value n Trend p -value n
SURVEY-WIDE -2.5 **** 203 +0.4 96 -0.2 186
STATES
Arizona +1.5 5 N/A N/A +2.8 5
Colorado -4.1 * 32 -9.6 ** 8 +2.6 *** 32
Kansas -8.3 * 10 -4.1 6 -1.2 7
New Mexico +0.3 36 +0.5 9 -1.1 35
Oklahoma +3.0 20 +9.3 * 8 +1.2 18
Texas -2.9 **** 99 +1.8 63 -0.9 89
PHYSIOGRAPHIC STRATA(2)
S. TX Brushlands -3.4 **** 21 +5.2 ** 16 -2.9 * 19
High Plains Border -0.8 15 +8.6 * 9 -2.5 * 11
High Plains -4.5 * 31 -8.8 *** 10 +2.5 *** 30
Edwards Plateau -7.0 **** 19 -4.8 * 14 -5.7 **** 17
Rolling Red Plains -0.9 19 -1.4 10 +0.6 18
Staked Plains -0.3 27 -0.8 11 -0.6 25
Chihuahuan Desert -1.7 29 +1.6 14 -0.5 28
Intermt. Grasslands -0.4 18 +2.5 3 -2.7 17
REGIONS
Central BBS Region -2.6 **** 149 +0.3 77 0.0 134
Western BBS Region -1.9 * 54 +1.8 19 -1.1 52
USFWS Region 2 -2.3 **** 160 +1.7 81 -0.9 * 147
USFWS Region 6 -4.4 * 43 -8.5 *** 15 +2.5 *** 39
(1) Without a significant p - value, there is no evidence that the trend is significantly different from zero (no
trend), and without sufficient sample size (BBS normally only calculates trends with a sample size of n > 14),
trend estimates are not reliable. A few results with n< 14 are presented here solely to document the limited
data available for some areas within Cassin’s Sparrow range.
(2) Some Physiographic Strata have recorded a few Cassin’s Sparrows, but the sample size is not large enough
to calculate a trend estimate - Coastal Prairies; Oaks and Prairies; Osage Plains; Rolling Red Prairies; and
Mexican Highlands.
When BBS data analysis is broken down into shorter time periods
(Sauer et al. 1997), however, the patterns rapidly break down (Table 3).
Survey-wide data for Cassin's Sparrow show no significant trends for
either the 1966 - 1979 or the 1980 - 1996 periods. At the state level
Colorado data show a significant decline for 1966 - 1979 and a very
significant increase for 1980 - 1996. Oklahoma data show a somewhat
significant increase for 1966 - 1979, and no significant pattern for 1980 -
1966. No other state data show any significant trends for either of these
Population Estimates and Trends
shorter time periods.
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 21
Figure 5. Annual population trend estimates for Cassin’s Sparrow from Breeding Bird Survey 1966 - 1996
(Sauer et al. 1997)
BBS TREND MAP 1966 - 1996
In spite of this variation in temporal trend patterns at larger scales,
much of which may be related to the great variation in annual
distribution of this species, several basic evaluation criteria can be
identified to search for potential problems at smaller scales, such as at
the physiographic stratum level. Patterns in which early (1966 - 1979)
trends were level or increasing and later (1980 - 1996) and overall
(1966 - 1996) trends are declining would appear to be of more concern
than patterns where early declines or level trends are followed by
increases. In addition, trends that show up in the core of Cassin's
Sparrow range are more likely to raise concern than patterns on the
periphery where annual variations in distributions may be greater.
Using these criteria identifies the trends in Texas (Table 3) as a
potential area of concern. We observed the most significant downward
trend in Cassin's Sparrow numbers over the entire BBS time period,
and no clear pattern in either of the shorter time periods. By looking at
the trend patterns in individual physiographic strata (Table 3) within
Texas, one can get an idea of where the problems might be. The most obvious
pattern of concern is in the Edwards Plateau where there is a somewhat
Population Estimates and Trends
22 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
significant downward pattern in 1969 - 1979, followed by highly significant,
steep declines in 1980 - 1996 and across the entire time period. Of similar
concern are the patterns in the South Texas Brushlands where there was a
significant increase from 1966 - 1979, then a somewhat significant decrease
from 1980 - 1996, and an overall highly significant decline for the entire time
period (1966 - 1996). Figure 4b shows annual indices of abundance for these
and other physiographic strata. There are not any significant or consistent
trends to raise concern in the other physiographic strata in Texas with large
enough sample sizes to analyze for Cassin's Sparrow trends (Rolling Red Plains;
Staked Plains; Chihuahuan Desert). From this information, it would appear that
the declines in Texas, and possibly the survey-wide trends for this species, are
being driven primarily by declines in the Edwards Plateau and the South Texas
Brushlands.
Applying the same criteria to New Mexico, which is also within the
core of the Cassin's Sparrow's range, the BBS data show no significant
or consistent trends over time and neither do any of the physiographic
strata within the state (Table 3).
Possible Explanation Historic Landscape Change in Texas - The Edwards Plateau of
of Trends central and west-central Texas (Figure 6) covers about 93,240 square
kilometers (Riskind and Diamond 1988). It has undergone significant
landscape-level habitat changes in recent history. Prior to settlement,
most of the Plateau was a fire-maintained savannah whose principal
woody species was live oak (Quercus fusiformis). Due to intense,
confined grazing which removes fuel and reduces water infiltration
rates, and the resulting decreased fire frequency, the Plateau has
experienced widespread expansion of woody plants, especially Ashe
juniper (Juniperus ashei), resulting in dense stands locally known as
"cedar brakes" (Fowler and Dunlap 1986; Riskind and Diamond 1988;
Taylor and Smeins 1994; Fuhlendorf et al. 1997). The result is a
transformation from grassland and oak savannah communities to a
woodland dominated by juniper, oak, and mesquite. It has been found
that Ashe juniper has the potential to produce nearly closed canopy
stands where composition and diversity of flora (such as herbaceous
ground cover) and fauna can be greatly altered (Fuhlendorf et al. 1997).
However, Fuhlendorf et al. (1997) did find that the influences of Ashe
juniper are dependent on factors including the size of the juniper trees
and long-term grazing or browsing history (with appropriate browse
levels controlling some of its spread and negative effects). There is no
evidence that Cassin's Sparrows use the cedar brake habitats. In
Maxwell's study (1979) in the northwestern part of the Edwards
Plateau, of the two habitat types in which another juniper species
(Juniperus pinchotii) was the dominant tree - juniper-liveoak savannah
and scrub oak shrubland - one Cassin's Sparrow was observed during
the breeding season and winter in one year in the former, and none
were observed in the latter. These numbers are small in comparison to
those reported by Maxwell in more preferred habitats, as described in
the "Habitat" section of this report.
Population Estimates and Trends
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 23
Figure 6. Map of Texas showing Edwards Plateau and South Texas Brushlands
physiographic strata.
In addition to the threats posed by woody encroachment, there has
been substantial suburban development in the eastern and southern
parts of the Edwards Plateau (B. Ortego pers. commun.; K. Bryan pers.
commun.), with the Plateau becoming more important as a recreational
and second home area (USFWS 1992; Fuhlendorf et al. 1997). Counties
on the eastern Edwards Plateau surrounding Austin and San Antonio,
as well as smaller cities such as San Marcos and New Braunfels, are
experiencing population growth rates several times greater than the
U.S. average and projections are that it will continue (USFWS 1990;
USFWS 1992).
The South Texas Brushlands encompass about 8 million ha in the
southern tip of Texas (Figure 6), just south of the Edwards Plateau. In
presettlement times, much of south Texas was covered by grassland
with scattered groves of thorn forest (Rappole et al. 1986). However, as
a result of a combination of fire reduction and grazing pressures, by the
early 1900's there had been a considerable change in the grasslands,
with an increase in cacti and woody species on upland sites, an increase
in annual grasses, and a decrease in perennial grasses (Rappole et al.
1986), so that south Texas is described now as semiarid brushland. In
comparison with the Edwards Plateau, in the South Texas Brushlands, the
Population Estimates and Trends
24 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
threats posed to Cassin's Sparrows may actually be related to destruction of
brushland through conversion to agriculture, urban development, and brush
control, as opposed to shrub encroachment. For example, since the 1920's in
the Lower Rio Grande Valley (LRGV) (the southernmost part of the South
Texas Brushlands), more than 95% of the original native brushland has been
converted to agriculture or urban use (Jahrsdoerfer and Leslie 1988).
Brushlands have been converted through mechanical clearing, pesticides, and
fire. Large-scale removal of brush began in the early 1900's. By the early
1930's extensive mechanized brush removal began, developing methods
including tractors pulling large cables, heavy chains, rolling choppers, root plows,
brush mowers, and tree grubbers (Inglis et al. 1986). In the Rio Grande Plain
alone, more than 3,000 ha of brush/year were destroyed from 1930 to 1948;
more than 21,000 ha/year from 1949 to 1954; and almost 20,000 ha/year from
1955 to 1959 (Davis and Spicer 1965). From 1940 to 1981, Texas landowners
treated an average of 600,000 ha annually to remove thorn forest (Welch 1982).
In comparison to brushlands cleared for agriculture, rangelands that are
managed for brush will at least eventually return to less diverse brushlands with
a grass component. However, most efforts lead to a control-regrowth cycle of
five to ten years (Davis and Spicer 1965), and the land is often planted to exotic
buffelgrass (Pennisetum ciliare ) to increase forage for livestock. This exotic
grass competes with native grass and forbs, greatly reducing habitat value for
birds, as well as other flora and fauna (Jahrsdoerfer and Leslie 1988). Finally,
urbanization poses an additional threat to the South Texas Brushlands. Using
the LRGV as an example again, human population has increased steadily since
the early 1900's, with the period from 1980 to 1990 expected to outgrow the
state average of 27% with a rate of 40% (Jahrsdoerfer and Leslie 1988).
HABITAT Although Cassin's Sparrows use slightly different habitats in different parts of
their range, the common denominator across all habitats seems to be that they
require both a grass component (usually short grass) and a shrub component.
The latter component may be actual shrub species [e.g., mesquite, sage
(Artemisia spp.), hackberry (Celtis spp.), rabbitbrush (Chrysothamnus spp.), or
oaks (Quercus spp.)] or other vegetative forms that approximate shrub
structure [e.g. yucca (Yucca spp.), paddle cacti (Opuntia spp.), ocotillo
(Opuntia spp.) or bunch-grasses] (Baicich and Harrison 1997; Rising 1996;
Williams and LeSassier 1968). The need for the structure provided by shrubs or
similar plants is related to the bird's need for perches from which to sing or
launch itself for its flight song and its frequent use of low shrubs for nest
placement. Schnase (1984) also noted that the mesquite thickets within
Cassin's Sparrow territories were distinctly preferred when fledglings were
present. It appears that relative proportions of grass and shrubs in acceptable
Cassin's Sparrow habitat cover a wide range from grassland habitats with a
very sparse distribution of shrubs to shrubland habitats with a grass cover
(JMR).
Population Estimates and Trends
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 25
See Appendix A for specific descriptions of the habitat used by Cassin's
Sparrows in individual states. See Appendix B for additional habitat descriptions
from primary references cited in sections below.
Specific research on habitat selection in Cassin's Sparrows has been limited and
much of it was conducted in southeastern Arizona habitats. This is worthy of
note since the application of these conclusions across the entire breeding range
of Cassin's Sparrow may not be warranted. The variation in habitats used
across their range is great, and Cassin's Sparrow response to habitat and
environmental change may also vary. Broad application of these results should
be done with care.
Breeding Season Results of studies conducted in semidesert grasslands in
Habitat Requirements southeastern Arizona at the Appleton-Whittell Research Ranch
(AWRR, also called the Audubon Research Ranch) and surrounding
land in Santa Cruz and Cochise Counties were consistent with the
"common denominators" mentioned above. A comparison of habitat
selection in three Aimophila sparrow species found that Cassin's
Sparrows were positively and significantly correlated with upland
mesquite grassland mesa habitat, as compared with sacaton
bottomlands, or ravine and slope habitats (Webb and Bock 1990). A
comparative study of grassland bird habitat preferences in the same
area found that Cassin's Sparrows were most strongly associated with
areas of high shrub canopy cover and density. They rarely occupied
plots with greater than 35% bare ground, and usually avoided areas
with less than 6% shrub canopy cover (Table 4). Compared to the
other species in the study, they preferred sites with more, taller grass
and less bare ground, and occurred more often near mesquite trees
(Bock and Webb 1984). See Appendix B for additional descriptions of
the upland grassland habitat at AWRR.
Table 4. Average characteristics of plots occupied by Cassin's Sparrows
(from Bock and Webb 1984).
Vegetative Characteristic Mean (SD)
Bare ground (%) 23.0 (±4.5)
Grass cover (%) 68.8 (±7.5)
Grass height (cm) 29.1 (±6.4)
Mean dist. (m) to 3 nearest mesquite trees 18.4 (±14.2)
Mesquite canopy (%) 03.7 (±3.3)
Shrub canopy (%) 10.3 (±5.5)
Shrub density (shrub/100 m2) 23.6 (±13.9)
Herb cover (%) 02.9 (±1.9)
Results from another set of studies on bird communities in
southeastern Arizona at the Santa Rita Experimental Range (SRER)
in Pima County might initially appear contradictory to those mentioned
above. In a comparison of avian communities in grassland habitat with
low mesquite density (called "grasslands") and habitats with high
mesquite density (called mesquite savannah), Maurer (1985) found that
Cassin's Sparrows were more common in the "grasslands" than in "mesquite
savannah" (Table 5). In a related paper focusing on habitat modeling, Maurer
Habitat
26 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
(1986) found that Cassin's Sparrows were positively associated with greater
ground cover and grasses typical of healthy ranges, and were negatively
associated with mesquite trees and grass species characteristic of poor range
conditions (e.g., Rothrock grama (Bouteloua rothrockii) - they tended to avoid
areas of high mesquite density and low grass cover. See Appendix B for
additional descriptions of the habitat at SRER.
Table 5. Densities of singing males per square km in two habitat types (from Maurer 1985).
Year Mesquite Savannah Grassland
1982 4.9 43.2
1983 34.5 71.2
A closer look at these papers indicates that they may not really be
contradictory. I believe that the AWRR and the SRER are located at
different places along a habitat continuum, with the AWRR found at
one end of the continuum described as upland grasslands with a low
density of large shrubs and mesquite, and the SRER at the other end
described as a desert shrubland with high mesquite densities and
varying amounts of grass ground cover. In this light, the combined
results may actually tell us more about Cassin's Sparrow habitat use,
rather than showing conflicting results. It is possible that at the
AWRR, since Cassin's Sparrows require a shrub component in their
habitat and very few shrubs were available, they preferred sites with
more mesquites. By comparison, at the SRER where plenty of
mesquite are available but Cassin's Sparrows also require a grass
component, the birds preferred sites with relatively lower mesquite
density. In fact, Maurer (1986) states that there is generally a negative
relationship between mesquite density and grass productivity at
SRER. In other words, Cassin's Sparrows require a shrub component
within grasslands, but if shrub density becomes too great, there is no
longer a grass component under/between the shrubs to meet the
species' other need. Although it is not possible to confirm this by
comparing shrub densities due to lack of compatible data in the two
manuscripts, scrutiny of the literature and personal conversations with
individuals familiar with these areas (D. Krueper pers. commun.; C.
Bock pers. commun.) support this hypothesis.
A study of breeding biology in south-central Texas found that all
Cassin's Sparrow territories included some combination of dense
mesquite thicket and open, grassy areas with scattered cacti and small,
shrub mesquite trees. An average of 28.4% of individual territories was
composed of mesquite thickets (Schnase 1984). Another study in the
same area (Maxwell 1979) compared bird densities from September
1975 to June 1977 in eight different plant communities - riparian forest,
juniper (Juniperus spp.)-live oak savannah, scrub oak shrubland,
bottom mesquite woodland, upland mesquite woodland, defoliated
mesquite shrubland, scrubby mesquite grassland, and grassland. He
found the highest breeding densities of Cassin's Sparrows in scrubby
mesquite grassland (33 birds per 40.4 ha in 1976 and 20 birds per 40.4 ha
in 1977). He described this habitat as having an absolute shrub density of 717/
ha, a relatively low foliage height diversity (0.540), the lowest percent vegetative
Habitat
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 27
cover (92%) of all sites, a foliar ground cover from 0.3 to 1.2 m of 22% and
from 1.2 to 3.1 m of 1%. They were also found in lower densities in bottomland
mesquite woodland (6 and 1/40.4 ha), upland mesquite woodland (7 and 8/40.4
ha), grassland (7 and 11/40.4 ha), and defoliated mesquite (7 and 11/40.4 ha).
Winter surveys found only a few Cassin's Sparrows in upland mesquite
woodland (2/40.4 ha in 1977), and juniper-live oak savannah (1/40.4 ha in 1977).
Maxwell states that Cassin's Sparrows are found from open grassland through
all stages of mesquite succession, being most common in scrubby mesquite
grasslands and, as mesquite increases in stature and density, Cassin's Sparrows
decrease in abundance and eventually abandon the site. See Appendix B for
additional descriptions of the habitat at these sites.
A study conducted in southern New Mexico and central Chihuahua
(Meents 1979) focused primarily on total bird community measures, but
provided some species-specific information. Cassin's Sparrows were
present on all four sites during the breeding season, but were most
abundant at the La Campana site in northern Chihuahua (mean birds
per roadside census = 74.0); values for the three southern New Mexico
sites were 14.5, 17.4 and 8.0 respectively. Although she did not evaluate
individual species associations with habitat, her characterizations of
vegetation at the four sites indicate that the La Campana site had
higher shrub cover - 8.4 % (± 5.6) - as compared with the other sites
which had 0.5 % (± 0.5); 0%; and 0% respectively. Percent grass and
forb cover appeared quite similar. This lack of shrub component at the
other sites may explain the smaller numbers of Cassin's Sparrows
found there during the breeding season. La Campana was the only site
where Cassin's Sparrows were found in the winter, and there in
extremely small numbers (mean birds per roadside census = 1.0). See
Appendix B for additional descriptions of the habitat at these sites.
A study of grassland bird habitat use on the Comanche National
Grasslands in Colorado found that sites where Cassin's Sparrows were
detected could be characterized by 27% bare ground, 14.8% shortgrass,
37.8% midgrass, 8.5% forbs, 2.4% cholla, 4.6% yucca, 0.9% low shrub (<1
m), and 4.1% tall shrub (>1 m) (Gillihan 1999).
Winter Habitat Very little research has been done to define the wintering habitat of
Requirements Cassin's Sparrow. Rising (1996) reports that in the winter they occur in
habitat very similar to their breeding habitat, but only in the southern
part of their U.S. range. Kaufman (1996) reports that in migration and
winter, they are found in pure grassland, brushy areas, and deserts. On
the wintering grounds in Texas, Cassin's Sparrows in desert areas seek
brushy draws and canyons, while in southern Texas they use prickly-pear
cactus (Opuntia sp.) and thick brush that dot savannahs
(Oberholser 1974). K. Bryan (pers. commun.) stated that Cassin's
Sparrows used a more limited set of habitat types in the winter in
Texas than they did while breeding. He felt they were using the
thickest, well-developed grassland with a shrub mosaic, and although
they were in the Trans-Pecos in the winter, they weren't using the
desert scrub that they did during breeding season. In southeastern Arizona,
Cassin's Sparrows occupy small grassy cienegas in lowland desert flats
Habitat
28 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
otherwise dominated by creosote bush (Larrea tridentata ). These cienegas
have dense grass coverage, with scattered mesquite and other shrubs (J.
Dunning pers. commun.).
A study of the size and structure of wintering avian communities in
seven habitat types in south Texas, found that Cassin's Sparrows occurred in
three habitat types -- open brushland (60% grass/forbs and 10% low shrubs),
dense brushland (50% grass/forbs and 30% low shrubs), and two-layered
brushland (50% grass/forbs, 30% low shrubs, and 10% tall shrubs). These
three habitat types could be defined as grassland/shrub habitats along a
continuum from grass with scattered patches of dense shrubs and mesquite, to
dense shrubland with a layer of taller mesquite and acacia shrubs above the
dominant shrub layer. Cassin's Sparrows were not found in grass-forb prairies
(100% grass/forbs), scrub grasslands (60% grass/forbs, 40% creeping brush
and no shrubs), oak woodlands, or riparian forest. (Emlen 1972). See Appendix
B for additional descriptions of the habitat at these sites.
A study of distribution patterns and habitat associations of wintering
grassland birds on the Mexican Plateau (Carter et al. 1997; Carter et al.
1998) found that Cassin's Sparrows were found on plots with more than
average shrub cover, bare ground, litter, and tall grass.
MANAGEMENT Because Cassin's Sparrows are very responsive to vegetative structure and
grass/shrub components of their habitat (see above), they are potentially
affected by a number of management practices including grazing and shrub
control. However, reaching any conclusion about the effects of various
management practices or environmental conditions on Cassin's Sparrows is
difficult due to the limited literature on this subject. As mentioned elsewhere in
this report, there have been several studies each on grazing, fire, and exotics
impact on Cassin's Sparrows and other grassland birds, but they have focused
on a small area that arguably could be called the periphery of the species' range
- southeastern Arizona. Therefore, any application of this information to other
parts of Cassin's Sparrow range should be done with caution.
See Appendix B for additional habitat descriptions from primary
references cited in sections below.
Grazing There are definite reasons to view the existing literature regarding
Cassin's Sparrow responses to grazing with caution. As Saab et al.
(1995) state, ". . . birds respond differently to livestock grazing in
different places. The same amount of grazing that can be used to create
ideal habitat for a species in a tallgrass prairie may be equally certain to
destroy that same species' habitat in a shortgrass steppe or semidesert
grassland. Therefore, management recommendations, derived from
(synthesized) data . . . should be tailored to the various sorts of
grasslands involved." In fact, they go on to single out Chihuahuan
Desert grasslands as an example of climatically stressed habitats
where environmental perturbations like grazing can result in conversion into
desert shrublands. Although the grasslands of southeastern Arizona cannot
technically be classified in this group, they certainly represent arid grasslands
that may be more susceptible to grazing impacts than Cassin's Sparrow habitat
Habitat
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 29
in other parts of its range. In light of the evidence of Cassin's Sparrow
distributional response to changes in precipitation, it seems likely that it is a
species for which a particular grazing regime or intensity may have very
different effects under different vegetation or precipitation conditions. It is
certainly the case that some habitats within the core of Cassin's Sparrow
habitat, such as eastern New Mexico, are almost all grazed, and yet continue to
support substantial Cassin's Sparrow populations (JMR).
The effects of grazing on Cassin's Sparrows can be addressed at
two levels. The most obvious is the direct and immediate effect
of grazing on the habitat structure preferred by Cassin's
Sparrows - reduction of vegetative height and density, or
changes in species composition through grazing preference. Most
of the information presented here falls in that category.
Knopf (1996), in his discussion of endemic grassland birds and their
distribution across grassland habitats in the west, places Cassin's
Sparrows at the mixed grass to mixed grass/shrub end of a
bareground-shortgrass-mixed grass-mixed/shrub habitat continuum,
and at the moderate-light-none end of an excessive-heavy-moderate-light-
none continuum representing historical (bison) grazing patterns.
He states that breeding habitats of Cassin's Sparrow were probably
outside the distribution of historically large herds of bison on the plains.
A study comparing breeding grassland bird use of southeastern
Arizona grasslands on grazed and ungrazed land (Bock and Webb 1984)
concluded that Cassin's Sparrows were excellent indicators of lightly
grazed or protected range, but only where shrubs or small trees are
present. Over two breeding seasons, Cassin's Sparrows were found
only on ungrazed sites (Table 6a). Both Cassin's Sparrows and
Grasshopper Sparrows were found to occupy areas with more and
taller grass cover and less bare ground than did Horned Larks
(Eremophila alpestris) and Lark Sparrows (Chondestes grammacus)
(Table 4).
Another study of grazed and ungrazed sites in the same area (AWRR
and an adjoining ranch) during the breeding and nonbreeding seasons
(Bock et al. 1984) found that Cassin's Sparrows were significantly more
abundant on ungrazed plots than on grazed plots in both breeding and
winter seasons (Table 6a). In fact, in both cases this species was only
found on ungrazed sites. The amounts of ground cover and shrub cover
were major determinants of the bird communities. The ungrazed plots
supported 45% more grass cover, a comparatively heterogeneous grass
community, and significantly more herbaceous cover than did grazed
plots (Table 6b). Woody plants were significantly more abundant
overall on ungrazed plots (although mesquite was uncommon throughout and did
not differ between treatments), and various individual shrub species were
denser and/or larger on the ungrazed plots (showing evidence of cattle browsing
on grazed plots). An extension of this study, including an additional site and
additional year of data, confirmed the above results (Tables 6a and 6b) –
although some Cassin's Sparrows were found on grazed plots, they were
Management
30 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
significantly more abundant on ungrazed upland grassland sites across seasons
(Bock and Bock 1988).
Table 6a. Results of three related studies comparing Cassin's Sparrow responses to grazing in
southeastern Arizona (Bock and Webb 1984; Bock et al. 1984; Bock and Bock 1988).
* significantly greater than other treatment P < 0.01; ** P < 0.001.
Source Ungrazed Grazed
1. Bock and Webb (1984)
Mean birds/plot
Breeding season 1981 (n = 9) 4.0 (±2.0)* 0
Breeding season 1982 (n = 14) 2.2 (±1.0)* 0
2. Bock et al. (1984)
Total birds/treatment
Breeding season (n = 2) 66** 0
Wintering season (n = 2) 42** 0
3. Bock and Bock (1988)
Total birds/treatment
Across seasons (n = 4) 209** 19
Table 6b. Results of two related studies describing effects of grazing on vegetative cover in
southeastern Arizona (Bock et al. 1984; Bock and Bock 1988). Asterisks (*) indicate
treatment significantly greater than other treatment * P < 0.05; ** P < 0.001
Source Vegetation Category Ungrazed Grazed
Bock et al. 1984 Percent Ground Cover
Grasses - total 80.4** 55.6
Herbs - total 12.0** 5.6
Woody plants - total 13.5 11.5
Bare ground 17.6 34.6**
Woody plant numbers
Total woody plants/plot 37.6** 9.5
Bock and Bock 1988 Percent Basal Area Cover
Grasses 52.0* 36.0
Herbs 13.0 10.0
Shrubs 6.0* 1.0
Bare ground 27.0 51.0**
An unpublished study of bird community responses to the removal of
cattle from the BLM's San Pedro Riparian National Conservation Area
in southeastern Arizona found that Cassin's Sparrow density increased
from 1986 (before cattle were removed), through 1987 (when they were
removed), to 1991. This response appears to be related to changes in
sacaton-mesquite habitats along the river following cattle exclusion -
foliage volume at several levels increased substantially during this time period
(D. Krueper written commun.).
Although not supported by particular documentation, Sutton (1967)
describes preferred Cassin's Sparrow habitat in Oklahoma variously as
"moderately" or "lightly" grazed sand prairie with a scattered shrub component.
Management
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 31
The second and related question regarding the effects of grazing is, what are
the more long-term effects of grazing on the landscape and subsequently on
Cassin's Sparrow habitat? The complicated nature of grassland ecology,
especially as it applies to different types of grasslands, is beyond the scope of
this document. However, in general it is recognized that the long-term result of
grazing in many southwestern grassland habitats is a significant reduction of fuel
loads and a subsequent reduction in the frequency and intensity of fire that,
historically, has controlled the invasion of woody shrubs into grassland
ecosystems (McPherson 1995; Wright and Bailey 1982). It is possible,
therefore, that in grassland habitats with insufficient shrub component to support
Cassin's Sparrows, grazing may result in increased shrub cover and therefore
increased Cassin's Sparrow habitat. In comparison, in shrubland habitat an
increase in shrub density that results in insufficient grass ground cover, may
reduce or degrade preferred Cassin's Sparrow habitat. Documenting such
connections would be extremely difficult, however, and caution should be
exercised in applying such ideas to management because of the potential mixed
impacts on other species.
Burning Most literature regarding Cassin's Sparrow responses to fire addresses
the short-term issue of its response to the direct, immediate effects of
fire on a habitat. A comparison of bird communities in burned and
unburned sacaton grasslands in southeastern Arizona (Bock and Bock
1978) found that total bird populations responded dramatically to
summer burns with large numbers of seed-eating birds, including
various sparrows, concentrating on the burns in the first two postburn
years. The effect of the fires on vegetation was to reduce sacaton cover
and greatly increase herb production through two postfire growing
seasons. Although this study did not conduct statistical analyses of
individual species numbers, the data presented for Cassin's Sparrow
numbers indicate this same pattern. The differences may have been
even more dramatic since data were collected throughout the year and
pooled to arrive at mean bird numbers. This method also precludes any
conclusions about seasonal differences in habitat use. A review of this
study (Bock and Bock 1988), using some different analyses, found that
Cassin's Sparrows were significantly more abundant on the burned
sacaton plots (P < 0.001) than on the unburned plots.
In comparison, a study comparing bird community response to wildfire
in native and exotic grasses (Bock and Bock 1992) presents a more
complicated picture that initially appears to contradict some of the
above results. However, although this study was also conducted at the
AWRR, it was conducted on upland mesa grasslands, which Cassin's
Sparrows prefer (Webb and Bock 1990), rather than the very different lowland,
sacaton grasslands studied in Bock and Bock (1978). A fire in July 1987
significantly reduced grass cover through two years on both native and exotic
sites, did not alter the proportion of exotic grass cover in the exotic plots,
reduced shrub cover, and resulted in an increase in herbaceous growth. Total
bird numbers in the fall increased dramatically on burned plots in both native and
exotic grasses for two postfire seasons. However, Cassin's Sparrows were
uncommon in the fall and did not show any significant responses to burns. For
breeding Cassin's Sparrows, results were clearest in native grasses, where
breeding Cassin's Sparrows were significantly less abundant on burned native
Management
32 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
grasslands for two postfire years (Table 7). It appears that during the breeding
season they preferred burned to unburned exotic grasses, but their response to
burns in exotic grasslands may have been confounded by an apparent reduction
in numbers of Cassin's on unburned exotic grasslands over the period of the
study (Bock and Bock 1992).
Table 7. Total numbers (means and SD) of birds recorded cumulatively on summer and winter
counts on 50 m diameter plots in burned and unburned native and exotic grassland in
southeastern Arizona - fire occurred July 1987 (Bock and Bock 1992). * values for burned
and unburned significantly different within grassland type P < 0.01; ** P < 0.001
Season Year Native grass Exotic grass
Burned Unburned Burned Unburned
(n =14) (n =14) (n =11) (n =11)
Breeding 1984-85 1.18 (±0.60) 1.2 9(±0.47) 0.46 (±0.52) 0.21 (±0.43)
(May-Aug.) 1987 0.00* 0.86 (±0.77) 0.00 0.14 (±0.36)
1988 0.55 (±0.52)** 1.86 (±0.77) 0.64 (±0.51)** 0.00
1989 1.55 (±0.93) 1.36 (±0.84) 0.91 (±0.54)** 0.00
1990 1.09 (±0.94) 1.29 (±0.61) 0.46 (±0.52)* 0.00
Winter
(Sept-Nov.) 1984 0.36 (±0.51) 0.29 (±0.61) 0.18 (±0.60) 0.00
1987 0.00 0.14 (±0.36) 0.09 (±0.30) 0.14 (±0.36)
1988 0.18 (±0.62) 0.43 (±0.65) 0.00 0.00
1989 0.09 (±0.30) 0.07 (±0.27) 0.00 0.00
1990 0.36 (±0.51) 0.29 (±0.47) 0.00 0.00
In a study of the effects of fire on grassland birds on the Buenos Aires
NWR, Gordon (In Press b) found that Cassin's Sparrows were
significantly more abundant on unburned plots (n = 3) than burned
plots (n = 3) in the first postburn year, even though there had been no
significant difference in abundance between plots previous to the burns.
During additional bird monitoring surveys conducted on the Buenos
Aires NWR (A. Flesch written commun.), data were collected on
sparrow use of plots with different fire histories. Flesch provided the
following observations, which are consistent with the pattern found by
Bock and Bock (1992) for breeding Cassin's Sparrows in native grasses.
Cassin's Sparrows were not as abundant on first-year postfire, and
perhaps second-year postfire plots. In 1997, no Cassin's were detected
on plots that had been burned that spring. In 1998, several singing
males were found on plots that had been burned 3.5 months earlier.
Flesch suggests that the presence of some Cassin's Sparrows on burned plots in
1998 was due to the unusually high rainfall received in late winter and early
spring 1998, resulting in more rapid grass regrowth.
The second level at which fire can affect Cassin's Sparrows is by its
long-term effect on habitat type. It is widely recognized that fire
interacts with other factors (e.g., soil, herbivory, vegetation type) to
restrict woody plant establishment in grasslands and that fire
suppression resulting from reductions in fuel loads, especially from
grazing, in the most extreme cases may result in transformation of
grassland to shrubland (McPherson 1995; Wright and Bailey 1982). Although
Cassin's Sparrows do require a shrub component in their habitat, dense woody
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U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 33
shrublands without a substantial grass cover provide less than ideal Cassin's
Sparrow habitat (Maurer 1985; Maurer 1986).
Conservation Reserve A study of the value of CRP lands to breeding nongame birds in the
Program Lands Texas Southern High Plains (southern part of the Panhandle) found,
(CRP) not surprisingly, that these plantings were valuable habitat for
grassland birds in a landscape dominated by farms and cropland
(Berthelesen and Smith 1995) – only two nongame bird nests were
found in crop fields. A comparison was made of bird use in the most
commonly established CRP cover types – blue grama (Bouteloua
gracilis)/side-oats grama (B. curtipendula); blue grama/Kleingrass
(Panicum coloratum); and blue grama/plains bluestem (Bothriochloa
ischaemum) mixtures. In one year, Cassin's Sparrow nest density/acre
was significantly higher in blue grama/side-oats grama mixtures than
in the other two. The second year showed no significant differences,
and nest densities between years in the blue grama/side-oats grama
mixtures were significantly different. Insufficient data precluded
comparison of nest success estimates between different CRP cover
types, but pooling did find a significant difference in nest success
estimates (using the Mayfield method) and clutch sizes for Cassin's
Sparrows between years.
Tamarisk Tamarisk, or salt cedar, is an exotic that was originally introduced into
the United States from Eurasia for ornamental and soil erosion control
purposes. It has escaped and spread throughout riparian habitats in
the Southwest, where it has demonstrated remarkable adaptability and
competitiveness with native plants under the arid, saline conditions
often found due to water control practices. It has resulted in many
management disagreements related to its wildlife benefits and the
potential impacts of control activities.
A study along the middle Pecos River in New Mexico, designed to
establish baseline data on the effects of tamarisk removal on breeding
bird communities (Livingston and Schemnitz 1995), found that Cassin's
Sparrows were most abundant in grasslands with mixed shrubs
(including tamarisk), and used this habitat as well as grasslands devoid
of tamarisk (in this case alkali sacaton grasslands) more than sites with
dense stands of tamarisk.
Another study of avian use of tamarisk on the middle Pecos River
Valley (Hunter et al. 1988) found that Cassin's Sparrows did not use
any of the riparian habitat types during the winter. However, in the
breeding season, a few Cassin's Sparrows were found using honey
mesquite (3/40 ha) and tamarisk (1/40 ha) habitat, where these species
provided a shrub component but minimal upper canopy and middle
canopy structure. They were not found in the two types of cottonwood-willow
habitats.
A more thorough study of avian habitat use along the Pecos River from
1979 to 1981 (Hildebrandt and Ohmart 1982) found more evidence
comparing Cassin's Sparrow use of tamarisk and other habitats.
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34 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Using density estimates from two summer periods - June to July, and August to
September - they found the following data for Cassin's Sparrows: no birds in
the densest two tamarisk-dominated habitats that provided substantial canopy
structure (substantial foliage from 4.6 to 7.6 m, and substantial foliage from 1.5
to 4.6 m respectively); six to seven birds per 40 ha for tamarisk habitat with little
foliage above 3 m; and 16 to 24 birds per 40 ha for sparse (little foliage above
1.5 m) tamarisk habitat. For comparison, they found 6 - 12 birds per 40 ha in
sparse four-winged saltbush habitat; 6 - 12 birds per 40 ha in sparse honey
mesquite habitat; two to seven birds per 40 ha in cleared communities
dominated by weeds and shrubs; and three to four birds per 40 ha in cleared
communities dominated by grass.
THREATS
Habitat Cassin's Sparrows are shrub-grassland specialists. The loss of
grasslands with a shrub component through conversion to agriculture,
suburban development, and desert scrublands (Welch 1982; Fowler and
Dunlap 1986; Jahrsdoerfer and Leslie 1988; Riskind and Diamond 1988;
Taylor and Smeins 1994; Fuhlendorf et al. 1997; U.S. Census Bureau
1999a) is the primary threat to Cassin's Sparrow habitat. Over-grazing
and poor rangeland management also contribute to the loss of Cassin's
Sparrow habitat (Bock et al. 1984; Bock and Bock 1988). Regulation of
Cassin's Sparrow populations could also be influenced by other factors
(e.g., habitat fragmentation, plot size, etc.) or by the same factors
occurring on wintering grounds or during migration. However, we
know little about these issues.
Overutilization for No evidence was found to suggest that direct use of this species for
Commercial, Recreational, commercial, recreational, scientific, or educational purposes poses a
Scientific, or Educational threat.
Purposes
Disease or Predation No records were found to suggest that disease is a particular threat to
Cassin's Sparrow populations.
There were few records regarding predation upon this species.
Williams and LeSassier (1968) note records of predation by a shrike
(Lanius spp.), snakes, and possibly red ants (a nest was discovered in
which nestlings were consumed by red ants although it was uncertain
whether they killed the nestlings or simply consumed them when they
were already dead). Schnase et al. (1991) cited indirect evidence that predation
may be high in Cassin's Sparrows; they reported that only 54% of the eggs laid
were successfully fledged by birds in their study. Cassin's Sparrows
occasionally enter rodent burrows to escape predation (Bowers and Dunning
1985). Montoya et al. (1997) studied avian predation and the diet of Aplomado
Falcons (Falco femoralis) in Mexico and found that, although Cassin's
Sparrows were a relatively common potential prey item in the area, they were
not found in falcon pellets or prey remains. They suggested that Cassin's
Sparrows might have been too small as prey items when the most common
available species and prey species were meadowlarks (Sturnella spp.).
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U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 35
Inadequacy of Existing Current regulations directed at protection of bird populations and
Regulatory Mechanisms individuals appear to provide adequate protection for Cassin's Sparrows in the
United States. It is protected under the Migratory Bird Treaty Act of 1918 in
the U.S. In Mexico the species is protected under the Convention for the
Protection of Migratory Birds and Game Mammals of 1936.
However, there is no regulatory protection for Cassin's Sparrow
breeding or wintering habitat in the United States or Mexico. There
are specific locations that are protected for a variety of uses and
wildlife resources (e.g., National Grasslands, state Wildlife
Management Areas, and other federal, state, and private land
holdings), but none of these are focused particularly on the needs of
Cassin's Sparrows, such as habitat. Incentive programs like the
Conservation Reserve Program (CRP) may provide suitable habitat for
Cassin's Sparrows if certain plantings are used (Berthelsen and Smith
1995), but there are no particular recommended management practices
provided at this time. One exception is a recent set of best
management practices, including some for Cassin's Sparrows, provided
by the Colorado Bird Observatory to the Comanche National
Grasslands in Colorado (Gillihan 1999).
Other Natural or
Manmade Factors Pesticides - Pesticides have not been documented as a threat.
Population size - Although there is little reliable information on
current population size, and the secretive nature and large seasonal
variations in local populations make drawing conclusions difficult,
populations are broadly believed to remain high in many parts of its
range. At this time, there is no evidence to suggest that small
population size itself poses a threat to this species' survival.
Nest Parasitism - Cassin's Sparrows are considered uncommon
cowbird hosts (Friedmann 1963). All records provided by Friedmann
(1963) were from Texas; he did acknowledge that it was possible that
Cassin's Sparrows seem to be an uncommon host because their nests
are so difficult to find and therefore evidence of parasitism is also rarely
documented. Kingery and Julian (1971) reported a Cassin's Sparrow
nest containing three Cassin's Sparrow eggs and one Brown-headed
Cowbird (Molothrus ater) egg on the Comanche National Grasslands in
Baca County, Colorado. Schnase (1984) observed three nests parasitized by
Brown-headed Cowbirds, each with four Cassin's Sparrow eggs and one or two
cowbird eggs; two were subsequently abandoned, and the third, from which the
cowbird egg was removed, fledged one young. Hunter and Howe (unpub.
man.) report a nest with two Cassin's Sparrow young and one cowbird that
fledged, and adults feeding fledgling cowbirds nearby at the same time, south of
Artesia, New Mexico in June 1980.
Suburban development - In light of Cassin's Sparrow requirements
for a relatively "natural" habitat composed of a mixture of grass and
shrubland, habitat degradation and loss due to suburban development
has the potential to negatively affect this species. Although not
specifically focused on Cassin's Sparrow habitat, two studies on the impact of
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36 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
suburban development on native bird communities in southern Arizona indicate
the general impact such activities have on southwestern bird communities. Both
found that, although higher bird densities were found on suburban sites, 52 to
72% of the total density was composed of three or four exotic species, and
certain native desert species were not present (Emlen 1974; Rosenberg et al.
1987). Suburban development usually results in an increase in landscaping with
exotic plants and a decrease in native plant species. Mills et al. (1989) found
that in Tucson, native bird species richness and densities of native territorial
species were strongly correlated with the volume of native plant species and
lacked any correlation with exotic plant species volume. These results suggest
that similar threats are faced by Cassin's Sparrows, since their habitat
preferences are not compatible with the normal habitats provided in suburban
development.
The suburban growth occurring in many parts of the southwestern
U.S. makes this a real threat. The U.S. Census Bureau (1996) projects
(using the middle series projections) that the resident population of the
United States will increase by approximately 26.3 % from 1996 to 2025.
In comparison, four of the states within the Cassin's Sparrow range
show U.S. Census Bureau projections (1999a) exceeding the national
average for the same time period: New Mexico (55.0 %); Arizona (52%);
Texas (45.2 %); and Colorado (38.5 %). The U.S. Census Bureau
recently released additional information about the growth rates in
specific cities from 1990 to 1998 (U.S. Census Bureau 1999b), stating
that smaller cities experienced the fastest growth. On average, cities
containing at least 10,000 people grew 6.6% between 1990 and 1998;
cities with populations of more than one million in 1998 grew by 3.5 %;
and cities with populations between 10,000 and 50,000 grew the fastest
(8.6 %). In comparison to these national averages, the following cities
within the Cassin's Sparrow range exceeded the national averages
(U.S. Census Bureau 1999c), exemplifying the potential habitat
destruction posed by the expected suburban sprawl related to this
growth: Laredo, TX (43%); New Braunfels, TX (33.6 %); Brownsville,
TX (28.8 %); McAllen, TX (27.1 %); Deming, NM (27.1%); Las Cruces,
NM (22%); Santa Fe, NM (20.1 %); El Paso, TX (19.3 %); Austin, TX
(17%); Sierra Vista, AZ (15.4 %); San Antonio, TX (14.1 %); Del Rio, TX
(14%); Nogales, AZ (13.1 %); Silver City, NM (12.9 %); Las Vegas, NM (11.8
%); Midland, TX (11.5 %); Tucson, AZ (10.8 %).
Exotic grasses - A study of the ecological effects of exotic lovegrasses
(Eragrostis sp.) on the AWRR (Bock et al. 1986; Bock and Bock 1988)
found that total bird abundance was significantly greater on native
grass sites than on exotic grass sites in both summer and winter.
During the breeding season Cassin's Sparrows were significantly more
abundant on native grass plots than on exotic plots. In winter the
pattern was the same but it was not statistically significant. The study
found that native grass cover, native herb canopy, herb species
richness, shrub density, and shrub canopy were significantly reduced on
plots dominated by exotic grasses. Total grasshopper numbers were
signficantly reduced on exotic plots, an important observation since
Cassin's Sparrows consume substantial numbers of grasshoppers in the
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U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 37
breeding season (Bock et al. 1986). A study on effects of burns on native and
exotic grassland communities (Bock and Bock 1992), found that Cassin's
Sparrows appeared to avoid both burned and unburned exotic lovegrasses
(Table 7).
Grazing - There is evidence that in the southwest portion of its range, Cassin's
Sparrows are negatively affected by grazing. However, there have been no
similar studies conducted in the core of its range or on the eastern edge to
determine whether the different vegetation and precipitation characteristics
found there result in different responses to grazing.
Burning - With a few exceptions (e.g., sacaton grasslands), Cassin's
Sparrows will avoid burned sites for one or two years, probably due to
the temporary reduction in grass and shrub structure. However, from
a long-term perspective, burning Cassin's Sparrow habitat may have a
positive effect if it results in less dense shrub cover and, if fire
management permits a mosaic of burned and unburned sites, in the
short-term Cassin's Sparrows will not be negatively affected.
Brush Control - In light of Cassin's Sparrow requirements for a shrub
component in their habitat, programs to promote brush or shrub
control to benefit grazing have a potential to negatively impact this
species. However, their response is probably dependent on the
particular circumstances in specific locations - complete shrub removal
would likely have a negative impact, while reduction in shrub density
may be beneficial in some circumstances.
Unfortunately, no studies were found that specifically described the
response of Cassin's Sparrows to brush control practices. However, on
the SRER (Maurer 1985; 1986), the sites described as having lower
mesquite density were sites where mesquite control efforts had been
implemented. It was these sites that were preferred by Cassin's
Sparrows (see description in Breeding Habitat section), suggesting that
in some cases, where the shrub component has become too dense for
optimal habitat, shrub control may benefit the species.
Oberholser (1974) provides some observations about the history of
brush control in Texas. He suggests that Cassin's Sparrow habitat has
actually increased since 1933 when the Soil Conservation Service began
to subsidize ranchers for brush control. By 1968 brush removal had
been attempted on much of this species' range in the western two
thirds of Texas. In cases where clearing was followed by a regrowth of
native grasses and a sprouting of young mesquites and low bushes,
Cassin's Sparrows have benefitted. However, more efficient
mechanical and chemical means of extirpating all woody vegetation
have been developed (Oberholser 1974) that are effective at much
larger scales than initial efforts. In addition, native grasses are being
replaced with foreign plants (Rising 1996; Oberholser 1974), posing new
threats to this species.
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38 U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000
Tamarisk - At least in some areas, where tamarisk habitat provides a shrub
component without a substantial canopy, it can be used by Cassin's Sparrows.
However, tamarisk habitat significance for Cassin's Sparrows is minimal since
tamarisk is a riparian species and Cassin's Sparrows are only found in a few
riparian habitats (e.g., bottomland sacaton grasslands), and it provides no threat
to Cassin's Sparrows.
ASSESSMENT
RECOMMENDATION
Recommendation The recommendations presented in this section have been jointly
on Current Status developed by the author and Stephanie L. Jones, USFWS Region 6
Nongame Migratory Bird Coordinator, and Bill Howe, USFWS
Region 2 Nongame Migratory Bird Coordinator, after review of the
contents of this document.
Existing information warrants neither a conclusion that the Cassin's
Sparrow range-wide population is in serious decline, nor dramatic
range-wide conservation measures. In fact, BBS data indicate that in
the core of its range in New Mexico it is very abundant and its
population is relatively stable. Therefore, we recommend no change in
Cassin's Sparrow official status at this time.
However, there is evidence of serious population declines in certain
parts of Texas (Edwards Plateau and South Texas Brushlands),
apparently due to major habitat loss and degradation. These declines
are associated with landscape-level habitat changes due to ongoing
woody encroachment, agricultural and suburban development, and
shrub control activities. Fluctuations in Cassin's Sparrow numbers
from year to year at a particular site appear to be associated with its
response to precipitation-related habitat conditions, and may confound
efforts to understand population trends. And, lack of information about
Cassin's Sparrow population trends in Mexico omits a substantial
portion of the species' range-wide population. Additional information
about population trends in the Edwards Plateau and South Texas
Brushlands as well as Mexico, and about causes of local, annual
population fluctuations, would be valuable in supplementing BBS data
and focusing more localized conservation efforts.
There also remain substantial gaps in our knowledge of this species' natural
history and ecology. There is no question that Cassin's Sparrows are adversely
affected by loss of shrubby grassland habitat to cropland or suburbs, but our
understanding of the effects of grazing and other rangeland management
activities on remaining habitat is less clear. There is evidence that Cassin's
Sparrows are negatively affected by heavy grazing in the most arid,
southwestern part of their breeding range, but additional comparative studies are
recommended to deter-mine their responses to types and levels of rangeland
management in the core of their range. In addition, there is very little
information available regarding their wintering or migration ecology, or their
breeding ecology in Mexico.
The threats to this species still exist. Based on current land use and
population growth patterns, the factors negatively affecting its popula-
Threats
U.S. Fish and Wildlife Service - Status Assessment and Conservation Plan Cassin’s Sparrow - March 2000 39
tions in the Edwards Plateau and South Texas Brushlands promise to increase
and spread, even into the core of its range. Therefore we recommend
maintaining the Cassin's Sparrow as a species of special concern, monitoring its
status, and promoting research and monitoring to expand our knowledge of its
ecology, status, and responses to management activities.
CONSERVATION Cassin's Sparrow is a member of a guild of species that are dependent on the
health and availability of shrubby, arid grasslands. Other species could benefit to
some degree from conservation efforts aimed at Cassin's Sparrow breeding
habitat, although details of their natural history, range, and habitat requirements
will vary. These other species include Grasshopper Sparrow, Loggerhead
Shrike (Lanius ludovicianus), Botteri's Sparrow, Western Meadowlark
(Sturnella neglecta ) and Eastern Meadowlark (S. magna). Many additional
species that rely on the same arid grasslands within the Cassin's Sparrow's
winter range would also benefit from conservation efforts there. These include
Vesper Sparrow, Baird's Sparrow, Savannah Sparrow, Lark Bunting
(Calamospiza melanocorys), Chestnut-collared Longspur (Calcarius
ornatus), and Brewer's Sparrow.
Cassin's Sparrow numbers fluctuate dramatically from year to year at
any particular site. This appears to be associated with its response to
fluctuating habitat conditions and climate, particularly precipitation.
These dramatic distributional changes suggest a species that might be
able to find suitable habitat wherever it exists within the breeding
range. However, if these fluctuations are indeed related to local and
regional weather conditions, then suitable habitat would need to be
provided throughout the breeding range to accommodate locally
variable precipitation and habitat conditions in any particular year.
These dynamics could make it difficult to designate and preserve
suitable habitat because Cassin's Sparrows may not be found in large
numbers on a particular site every year, except in the core of its range
in eastern New Mexico and western Texas.
Long term conservation of Cassin's Sparrows is linked to conservation of arid
grassland ecosystems, on both their breeding and wintering ranges. The limited
information available regarding Cassin's Sparrow response to various habitat
management regimes makes it difficult to recommend broad conservation
actions. However, some general recommendations can be made, based on
what we know, to help direct research to provide additional information.
l In the southwest portion of their range (southeastern Arizona
and southwestern New Mexico), where there is evidence that
heavy grazing can have detrimental effects on Cassin's
Sparrow habitat, removing cattle or at least reducing grazing
pressure will promote the preferred combination of shrub and
grass structure on Cassin's Sparrow habitat. Even
throughout the rest of its range, we do know that management