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Background and Justification

Grasslands are recognized by many as the most imperiled ecosystem worldwide (Vickery et al. 1999). The avian assemblages associated with grasslands also are at risk—grassland bird populations have shown steeper, more consistent, and more geographically widespread declines than any other guild of North American species (Department of the Interior 1996). Breeding Bird Survey data from 1966-1993 indicate that almost 70 percent of 29 grassland bird species adequately surveyed by BBS data had negative population trends; more than half of these were statistically significant.

In addition to range-wide population declines, the distribution and abundance of many grassland species are highly variable in space and time (Igl and Johnson 1995), which complicates conservation plans for grassland bird species. At both local and range-wide scales, variation in numbers from year to year may be driven by (1) climate patterns, which may significantly alter vegetation characteristics of the site and hence habitat cues used by birds in selecting breeding territories (Igl and Johnson 1995, Price 1995); (2) changes in the habitat caused by management actions or natural disturbances; (3) success of birds in raising young at that location in previous years, which may influence return rates and hence population stability at a site; or (4) changes in landscape structure caused by agriculture, urban sprawl, or other human activities. The relative importance of each of these factors has not been well established for grassland species, yet such knowledge is crucial to understanding patterns of range-wide population declines and local-scale fluctuations in grassland bird populations.

In an attempt to reverse population declines of grassland birds, the management concept of Bird Conservation Areas (BCA's) was suggested as a means to conserve grassland songbird populations (Pashley and Fitzgerald 1996). The notion behind BCA's is that core areas of quality habitat (such as native prairies) that are isolated from hostile habitats (such as woody vegetation) will result in reproductive rates sufficient to maintain population levels of breeding birds (Henderson and Sample 1995). The BCA concept implies that the value of high-quality core areas depends on the habitat composition of the landscape matrix in which the core areas are embedded. This concept is being promoted despite the absence of data that validate its usefulness in maintaining viable populations of grassland songbirds.

Moreover, the U.S. Department of the Interior (DOI) Conservation Strategy for declining birds in grassland ecosystems (DOI 1996) calls for information on effects of habitat and landscape features on population viability of grassland birds. High-priority information needs identified by the DOI include effects of habitat structure and composition on avian communities, and effects of landscape context (e.g., patch distribution, surrounding land use, and proximity to hostile environments) on avian numbers and nesting success. Furthermore, factors associated with highly variable population numbers (climate, habitat changes, nesting success) are needed to determine causes of population stability or instability over time. This information is critical for developing long-term conservation objectives that will benefit grassland birds, but is lacking for many grassland bird species.

The BCA concept was proposed by the Midwest Working Group of Partners In Flight and supported also by the Prairie Pothole Joint Venture. It was included in the draft of the Landbird Conservation Plan for Physiographic Area 40: the Northern Tallgrass Prairie. This evaluation of the BCA concept is intended to determine whether BCA's in the northern tallgrass prairie do, in fact, meet their intended objectives. The effort addresses needs identified in the Landbird Conservation Plan by evaluating its assumptions.

For this evaluation, we are considering native prairie (parts of which may have been restored) to be high quality habitat; heavily wooded vegetation, which can harbor high numbers of predators and brood parasites, to constitute hostile habitat; and small-grain and hay fields to be neutral habitats.

Objectives

1. To estimate distribution, abundance, and reproductive success of grassland bird species in large and small core habitats embedded within hostile and neutral landscape matrices.
2. To estimate between-year site fidelity of grassland songbirds and factors that influence site fidelity.

Study Areas

In 1999 the study was conducted in three areas in the northern tallgrass prairie: (1) east of Moorhead, MN, in Becker, Mahnomen, and Clay counties; (2) east of Crookston, MN, in Polk County; and (3) in southeastern North Dakota at Sheyenne National Grassland in Richland and Ransom counties. Study sites include tracts owned by the U.S. Fish and Wildlife Service, U.S. Forest Service, Minnesota Department of Natural Resources, and The Nature Conservancy (Table 1).

Methods

Study Design

We are using a two-way factorial experimental design that will address three major questions: (1) Does size of core habitat influence density and nesting success of birds? (2) Does landscape matrix (extent of woody vegetation surrounding the core habitat) influence density and nesting success of birds? and (3) Do landscape matrix and size show interactive effects? Main effects in the design are habitat size and landscape matrix, with several replicate plots within each size*landscape combination. In 1999, bird abundance was assessed on 45 study plots. All study plots were within native or restored prairie of similar vegetation structure and composition and varied between 1.5 and 16 ha in size. Nesting success was investigated in 30 of the 45 study plots (Table 1).

In total, we established 22 study plots within core areas that are "small" in size (<50 ha), and 23 study plots within core areas that are "large" in size (>250 ha). We searched for nests in 15 study plots within small core areas and in 13 study plots within large core areas. Differences in abundance and reproductive measures between large and small core areas will be referred to as "main effects of size." Twenty-two study plots (12 small and 10 large) were established within hostile landscapes. Hostile landscapes include landscapes that contain large areas of woodland habitat within 5 km of the core habitat. Twenty-three study plots (10 small and 13 large) were established within neutral landscapes. Neutral landscapes include landscapes that consist of habitats that are thought to have little or no impact on bird populations within the core areas, such as small-grain fields, hay-meadows, and Conservation Reserve Program fields. Differences in abundance and reproductive measures between hostile and neutral landscapes will be referred to as "main effects of landscape composition." Interactive effects between core habitat size and landscape matrix also will be examined.

Field methods

On each study plot, we measured vegetation characteristics and bird abundance. Nesting success was investigated on a subset of the study plots (Table 1). Study plots were marked with flags or wooden laths at 50-m intervals along transects that were 100 m apart. Vegetation was assessed at 10-34 measuring points within each study plot, systematically located throughout each plot. The number of measuring points taken within a plot varied with the size of the study plot. Vegetation was measured once, in early to mid July. Measurements included vegetation height, percentage cover by growth form (grass, forb, woody, bare ground, litter, and standing residual) based on a 20×50 cm Daubenmire frame, height-density (Robel readings), number of small (<30 cm tall) and large (>30 cm tall) woody stems, and litter depth. Vegetation characteristics in each study plot were evaluated to determine the associations between densities of each species and habitat characteristics, local (patch size) features, and landscape features.

Abundance of breeding birds of all species was determined on each study plot by strip-transect censuses (Stewart and Kantrud 1972). Censuses were conducted twice between 23 May and 27 June 1999 by Jill Dechant, except that Longspur and Plum plots were censused only once. The maximum count of a species was used to determine density (number of males/100 ha).

Reproductive success of birds was assessed by searching for nests and monitoring eggs and young until fledging. The observers located nests by walking through fields with or without flushing-sticks and looking for nests after flushing or observing birds. Nests were marked with a flag 5 m to the north and were revisited every 3 days to ascertain its status and the incidence of brood parasitism. Nest success was determined using the Mayfield method (Mayfield 1961). A nest was considered successful if it fledged at least one young of the parental species, and it was considered parasitized if it contained at least one cowbird egg or chick. We focused our nest searching efforts on three species: Clay-colored Sparrow, Savannah Sparrow, and Bobolink, but monitored nests of any other species we found..

Nest vegetation was characterized within one week after activity at a nest had ceased. Vegetation was measured at five areas near each nest: directly at the nest and at a distance of 0.5 m from the nest in each cardinal direction. At each of the five points we measured vegetation in the same manner as described above for plot vegetation. Vegetation characteristics at the nest were evaluated to determine the associations between reproductive success by species and microhabitat (vegetation), local (patch size), and landscape features.

Four of the 45 study plots (two plots in large core areas surrounded by neutral landscape, and two plots in small core areas surrounded by hostile landscape) were designated as intensive sampling plots. On these we captured and marked birds to assess factors associated with population stability at a local site over time, again focusing on Clay-colored Sparrows, Savannah Sparrows, and Bobolinks. Birds were banded with an aluminum federal band, and a combination of three color bands. The four sites were monitored throughout the season to determine if any of the 237 individuals banded in 1998 returned in 1999. Unbanded birds nesting on the plot were then targeted for banding. We then focused on monitoring banded birds to determine their seasonal reproductive success and movements within a plot. The goal of the intensive-sampling plots is thus to evaluate the number of young fledged per year and site fidelity for each adult of the three focal species. Site fidelity was measured in terms of returning to a site between years.

Results

We recorded 53 species of birds on our study sites (Table 2). The four most common species were Savannah Sparrow, Bobolink, Le Conte's Sparrow (Ammodramus leconteii), and Clay-colored Sparrow. Sedge Wrens (Cistothorus platensis), Le Conte's Sparrows, and Savannah Sparrows generally occurred in each patch size in relatively high densities, but appeared to have higher densities in neutral landscapes. In addition, Le Conte's Sparrows and Savannah Sparrows seemed to favor large prairie patches at Sheyenne National Grassland. Clay-colored Sparrows preferred hostile to neutral landscapes, and small patches to large patches, in each region. Bobolinks seemed to prefer large prairie patches in the Crookston region and neutral landscapes in the Glyndon region, whereas their distribution at Sheyenne National Grassland seemed to be independent of patch size and landscape. The only grassland-nesting species that were restricted to large plots were Greater Prairie-Chicken (Tympanuchus cupido) and Northern Harrier (Circus cyaneus).

Species composition differed slightly among the three regions (Table 2). Some species were detected in only one of the three regions, such as some duck species, and Lark (Chondestes grammacus), Field (Spizella pusilla), and Chipping (Spizella passerina) sparrows, which were found only at Sheyenne National Grassland. Further, species' densities varied among regions (Table 2); for example, Savannah and Le Conte's sparrows reached highest densities in the Crookston region, whereas Western Meadowlarks (Sturnella neglecta) and Grasshopper Sparrows (Ammodramus savannarum) were recorded most frequently at Sheyenne National Grassland. Bobolinks and Clay-colored Sparrows had similar densities across the three regions.

We found 795 nests of 34 species (Table 3). Compared to 1998, where we found 293 nests of 19 species, both the number of nests and the number of species were much greater. The greater total number of nests found was probably due to the addition of study plots and the increased number of field assistants. The greater number of species was partly due to the inclusion of Sheyenne National Grassland, where nests of eight species were found that were not detected on the Minnesota sites [(Gadwall (Anas strepera), Northern Shoveler (Anas clypata), Wilson's Phalarope (Phalaropus tricolor), American Bittern (Botaurus lentiginosus), and Lark, Field, Chipping and Vesper (Pooecetus gramineus) sparrows].

Most of the nests found belonged to the three focal species, Clay-colored Sparrow, Savannah Sparrow, and Bobolink (Table 3). Their nesting success varied greatly among regions, with no consistent pattern in relation to patch size and landscape (Table 4). It appeared that Clay-colored Sparrows had lower nesting success in hostile landscapes than in neutral landscapes, whereas patch size had no consistent effect on its nesting success. Nesting success in Savannah Sparrows and Bobolinks did not seem to be affected by either patch size or landscape structure (Table 4).

Cowbird parasitism was low, with 6.7% (40/598) of all grassland passerine nests parasitized. The most heavily parasitized species were Brewer's Blackbird (Euphagus cyanocephalus; 45%), Bobolink (16%), and Western Meadowlark (9.4%). All other grassland passerines had parasitism rates below 5%, even though in a single region parasitism rates were higher for some species (Table 4).

We color-banded 239 birds: 106 Clay-colored Sparrows, 101 Savannah Sparrows, and 32 Bobolinks. In addition, we found 72 nests for which at least one of the two parents was banded (30 Clay-colored Sparrows, 36 Savannah Sparrow, 6 Bobolinks). Nineteen of these banded birds had been banded in 1998: three Clay-colored Sparrows (two females and one male) and 16 Savannah Sparrows (10 males and 6 females). For four pairs of Savannah Sparrows, both male and female had returned from the previous year. Two birds were observed to have two broods during the field season. One male Clay-colored Sparrow paired with another female after his first clutch was depredated during the nestling stage. One female Savannah Sparrow was double-brooded: after successfully rearing four young with an unbanded male, she started a second brood with a banded male 40 days after the first nest was initiated. Her second brood was depredated in the nestling stage.

Discussion and Future Plans

In 1999 we found more than 700 passerine bird nests, including 519 nests of the three focal species (Table 3). Of these nests, 72 belonged to pairs of which at least one parent was banded. We were able to follow these banded birds throughout the season and identify any second broods. Because of the large number of nests found for certain species, we will be able to investigate nest habitat preferences based on vegetation measurements.

The addition of Sheyenne National Grassland was beneficial in that it increased the sample size of census plots and nests found. While two years of data collection are inadequate to definitively conclude how grassland-nesting birds respond to patch size and landscape structure, our preliminary results are promising. Because variation in avian density and nesting success is high between years, among plots, and among species, at least one other field season is necessary to discern trends.

The employment of crew leaders was highly beneficial to the project as can be seen by the increased number of nests found in 1999. For the upcoming field season we will therefore again employ individuals with expertise in nest-finding as crew leaders. Unfortunately in 1999 we did not have sufficient staff to search for marked birds off the study plots. If resources are available in the next year, we hope to employ one additional person for the intensive study crew to conduct resightings on and off the study plots.

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Acknowledgments

Primary funding for the second year of the evaluation was provided by the U.S. Geological Survey, through the Northern Prairie Wildlife Research Center, and the U.S. Fish and Wildlife Service, Region 3. Additional funding was provided by the U.S. Fish and Wildlife Service, Region 6, and by the Minnesota Nongame Wildlife Tax Checkoff, the Reinvest In Minnesota Program, and the Minnesota Chapter of The Nature Conservancy through the Minnesota Department of Natural Resources.

Access to study areas was kindly granted by The Nature Conservancy, U.S. Fish and Wildlife Service, U.S. Forest Service, and Minnesota Department of Natural Resources (Division of Fish and Wildlife and Division of Parks and Recreation).

We appreciate efforts made by the field assistants: Daniel Cariveau, Wade Cooper, Christine Dahlin, David Grosshuesch, Shawn Hawks, Dylan Maddox, Angela Mundt, Jason Scott, Emily Spinler, Valerie Steen, Corinne Wilkerson, and David Woodward. Peter Jones was a tremendous help in getting the mistnetting started. Thanks to the volunteer work by Harold Duebbert, Brandi Mansfield, and Maurice and Laura Vial. We are indebted for generous assistance with housing and other logistics to Mary Soler, Paul Soler, and Rick Julian of the U.S. Fish and Wildlife Service; Brian Winter, Sonia Winter, and Gordon Yalch of The Nature Conservancy; and Bryan Stotts of the U.S. Forest Service.

We are grateful to Stephen J. Lewis of the U.S. Fish and Wildlife Service for his continuing support of this evaluation.

Jane Austin, David Fellows, Pamela Pietz, and Marsha Sovada of the Northern Prairie Wildlife Research Center graciously loaned equipment and supplies. Betty Euliss assisted the project in numerous ways.

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Literature Cited

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