Breeding Birds of the Platte River Valley
Biogeographic Distribution of Breeding Birds
The central Great Plains of North America are at a biological crossroads for many breeding bird species. Because of its east-west orientation, the Platte River Valley has accommodated the movement of several closely related forms that typically have an "eastern" or "western" distribution. The area of range overlap, and the resultant integration of closely allied species has been the subject of several studies hybridization. Groups receiving the greatest amount of attention thus far include black-headed and rose-breasted grosbeaks (Kroodsma 1970, Kroodsma 1974), indigo and lazuli buntings (Kroodsma 1970), eastern and western meadowlark (Lanyon 1957), and Baltimore and Bullock's orioles (Sibley and Short 1964, Rising 1983).
Cook (1969) found a decrease in the number of breeding bird species progressing southward from central Canada. One explanation for decreasing numbers of species is found in examination of glacial history. During the pre-Pleistocene, much of the central United States was covered with tropical or sub-tropical forests. Advancing ice sheets caused species to retreat to the south and west, away from the central regions. At the close of the last glacial epoch, breeding bird distributions were influenced not only by the lack of species in response to glacial-period movements, but also by the increasing area of grasslands forming in the rain shadow of the Rocky Mountains.
The grassland biome is relatively depauperate of breeding bird species compared with more structurally complex habitats such as forests (Kantrud and Kologiski 1982). Thus, the early developmental history of the central United States avifauna was influenced by the preponderance of species adapted to the grassland biome. Peterson (1975) analyzed data from the North American Breeding Bird Survey (BBS) and ranked the biogeographic zones used to stratify the continent for the BBS by the average number of species observed per route. The High Plains Border (eastern half of our study area) ranked 33rd out of 56 strata, and the High Plains (western half of our study area) ranked 46.
Examination of species diversity among the 56 North American strata revealed that the High Plains Border ranked 37, and the High Plains ranked 47. Generally, the species diversity indices for these two strata suggest that although there was a moderate number of species available, their population levels were influenced by the preponderance of a few species in the sample. Among the species recorded on most BBS routes in the study area, red-winged blackbird, common grackle, European starling, and house sparrow were typically the four most numerous species.
Peterson (1975) went on to examine latitudinal stratification on the continent. A positive relationship was found between the average number of species recorded and the degrees of latitude north from southern Texas. Peterson concluded that the latitudinal stratification he found was influenced by the more heterogeneous landscape in the north, which attracts and supports more species than in areas farther south.
We find Peterson's (1975) conclusion to be in general agreement with our analysis. Because of its location at the center of the Great Plains, the Platte River Valley is far removed from the principal nesting areas for many bird species. For instance, the study area is about 650 km east of the coniferous forests of the Rocky Mountains. Although the Missouri River riparian forests are about 200 km from the eastern edge of our study area, the large contiguous eastern deciduous forest with its myriad of warblers, thrushes, and flycatchers are 800 km farther removed. The Platte River Valley is about 350 km south of the northern Great Plains prairie and wetland habitats, and about 1500 km north of the south Texas brushlands, and the species with typically Mexican or southwestern distribution that exist there. Although small outliers of most of these biomes exist in various regions of the Platte River Valley, and thus contribute to the overall uniqueness of the area, in general the lack of extensive areas of these habitats undoubtedly contributes to the overall lack of diversity of breeding bird species throughout the study area.
Examination of the zoogeographic distribution and faunal affinities of the breeding avifauna of the Platte River Valley, reveals that the majority of species are associated with woodlands (47.6%) and are of Pandemic origin (44.1%) (Table 5). Terms used in this analysis are from Johnsgard (1979) including:
Pandemic: a species having a large, continuous or disruptive distribution pattern not clearly associated with major vegetation types.
Endemic: a species largely limited to the grasslands and wetlands of the Great Plains.
Eastern: a species with a breeding distribution generally associated with areas or habitats east or southeast of the study area.
Northern: a species having a breeding distribution associated with areas or habitats north or northeast of the study area.
Western: a species having a breeding distribution associated with major areas or habitats west or northwest of the study area.
Southern: a species having a breeding distribution associated with major habitats south or southwest of the study area.
Introduced: a species added to the local avifauna as a result of humans.
The habitat affinities of individual species are described as:
Forested: a species having a distribution associated with native deciduous woodlands or earlier wooded successional stages.
Grassland and Xeric Shrub: a species having a distribution generally associated with native grasslands or introduced grass species; xeric shrub species are associated with arid or shrub-dominated vegetation types.
Aquatic: a species having a distribution associated with natural basin wetlands, rivers, or reservoirs and lakes.
Miscellaneous: a species having a breeding distribution not specifically associated with any of the above-mentioned vegetation types.
Most Pandemic species have forest affinities. The preponderance of eastern species in the breeding population suggests that the Rocky Mountain montane habitats, coupled with the wide expanse of arid grasslands are an effective barrier for the eastward expansion of western species. The virtual lack of northern and southern species with woodland affinities relates principally to the lack of northern coniferous communities and arid brushland communities in south-central Nebraska.
The influence of typical western species in the avifauna is especially pronounced among grassland birds, which make up about 44% of the western avifauna. The large number of grassland species reflects the importance of the prairie in contributing to the Platte River avifauna. Among aquatic species, about 59% are of Pandemic origin, whereas only about 6% are of northern origin. The aquatic species nesting in the study area are principally waterfowl and rails. Most waterfowl species considered here are dabbling ducks with wide-ranging distributions. The lack of a northern component among aquatic species reflects the lack of boreal-nesting waterfowl and also many of the common shorebirds including marbled godwit (Limosa fedoa)and the Calidris sandpipers.
Although located at about 40 degrees north latitude, the Platte River Valley avifauna is relatively depauperate of typical southern species (2.1%). Among the species in our study area, only the scissor-tailed flycather, Chihuahuan raven, great-tailed grackle, and blue grosbeak are of southern origin. There are scattered, sporadic nest records of the flycatcher and raven. The great-tailed grackle is a relatively recent addition to the nesting avifauna (Faanes and Norling 1979). Only the blue grosbeak is of regular occurrence as a nesting species throughout the study area.
The virtual lack of southern nesting species at this latitude appears to relate to past glacial activity which forced many species into southern refugia. Apparently the time span since the close of the last glacial epoch has not been sufficient to allow the colonization of the region by additional southern species. The absence of xeric shrub and brushland habitats throughout the study area appears to also have a pronounced influence on southern species colonization of the Platte River Valley. Perhaps the nearest large area supporting species typical of southern xeric habitats is along the Cimmaron National Grasslands in Morton County, Kansas, about 400 km south of North Platte, Nebraska.
The recent nesting-season sightings of species typical of southern habitats such as Mississippi kite, snowy plover, and white-faced ibis coupled with the rapid colonization of the study area by the great-tailed grackle, may be a signal of future movement of additional southern species into the study area.
Data gathered from the North American Breeding Bird Survey (BBS) (Bystrak 1981) provide an effective index to bird population levels, both temporal and spatial. Therefore, we analyzed data from 7 BBS routes in our study area (Fig. 4) to seek differences in relative abundance among species. These data suggest striking differences in relative abundance of some sources across an apparent east-west environmental gradient (Table 6).
Data for ring-necked pheasant suggest their relative abundance is patchy throughout the study area, although occurring in greatest numbers in northern Lincoln County. The distribution of northern bobwhite is equally patchy, although largest numbers are in the east and in areas supporting a heterogeneous mixture of native grasslands, woodlands, and cropland.
Among shorebirds, the killdeer occurs in greatest relative abundance in the western counties that support extensive areas of native grassland and fallow fields. Upland sandpiper occurs in greatest abundance along the route of the Kearney BBS, which traverses extensive areas of wet meadow habitats adjacent to the Platte River. The long-billed curlew is absent east of 100 degrees W. longitude.
Numbers of chimney swift are more heavily weighted toward the eastern half of the study area, generally east of 100 degrees W. longitude. The eastern half also contains more urban-residential areas which support chimneys providing suitable nest sites. The distribution of red-headed woodpecker abundance grades slowly from east to west. Much of the abundance of the species appears to be related not only to geographic location, but also to the presence of dead trees resulting from flooding and dutch elm disease. Northern flickers, on the other hand, appear to be most abundant in the central third of the Platte River Valley. Previous investigations (Short 1961), have shown the Platte River Valley to be an important hybridization zone for the yellow-shafted and red-shafted subspecies of northern flicker.
Relative abundance of the western kingbird and eastern kingbird appears to be similar except that the eastern kingbird occurs in slightly larger numbers west of 100 degrees W. longitude. Horned lark is also more abundant west of 100 degrees W. longitude. This pattern may be a reflection of actual biogeographic preference for western climate and physiography. However, habitat may be an important influence as well. Much of the uplands east of 100 degrees W. longitude has been converted to corn production. Our breeding bird population data reveal that corn provides unsuitable habitat for many bird species because of its rapid upward growth after emergence from the ground. Lower numbers of horned larks may be a reflection of habitat in addition to biogeographic affinities.
Among the Corvidae, blue jay numbers are largest in the east, which coincides with the eastern woodland affinity of this species. Black-billed magpies occur in largest numbers in the west. The American crow population is largest in the eastern half of the study area. American crow numbers probably reflect the larger number of farmsteads, shelterbelts, and feedlots here which crows use extensively for feeding, nesting and roosting.
Black-capped chickadee abundance is greatest in the eastern half of the study area, reflecting the general eastern affinity of this woodland species. House wrens are also most abundant in the east. Eastern bluebirds become virtually nonexistent west of 100 degrees W. longitude, and American robin numbers also decline west of that longitude. Gray catbird distribution appears to be related to the location of Riparian Forests along the Platte River, but northern mockingbirds are most prevalent in the west. The latter may reflect colonization of the Platte River by southwestern populations of this ubiquitous species of the southern United States.
The center of abundance of the brown thrasher was in the western half of the study area. The occurrence of large numbers of brown thrashers in the west may be a reflection of habitat stability because this species is typically associated with eastern or southeastern scrub habitats. The intensive land use in the eastern half of the study area may be affecting brown thrasher distribution and abundance. Loggerhead shrike numbers are also greater in the west. This species, too, may be responding to variations in land use intensity further east, although sizeable numbers of loggerhead shrikes occur in the xeric habitats southwest of the study area.
The abundance of European starling in the eastern half of the study area reflects habitat preferences and availability. Not only are dead trees supporting suitable nest sites more numerous in the east, but cattle feedlots and other preferred starling feeding sites are more prevalent in the east. The first European starling in Nebraska was recorded at Hastings, Adams County, in 1937 (Hudson 1938), thus the eastern population has also been established longer.
Common yellowthroat abundance reflects the eastern affinity of this species. Northern cardinal numbers are also related to the eastern origin of this species, and may reflect the slow colonization of western habitats along the Platte River. Blue grosbeak numbers reflect the southern xericorigin of this species which becomes sporadic east of 100 degrees W. longitude. Indigo bunting, an eastern deciduous species, occurs in sporadic numbers adjacent to the Platte River. Surprisingly, relative abundance appears to increase as the species approaches the zone of overlap with the congeneric lazuli bunting. Dickcissel abundance follows the southeastern and southern origin of this grassland species.
Among the Emberizid sparrows, relative abundance of field sparrows appears to be greatest west of 100 degrees W. longitude. Areas supporting largest numbers of field sparrows are associated with the mixed forests occurring in the highly dissected mixed prairie region of southern Lincoln County. Lark sparrow abundance reflects the western origin of this population and also the species apparent preference for shrub-dominated native grasslands supporting soapweed yucca. Lark bunting abundance increases rapidly as the 100 degrees W. longitude line is approached. Largest numbers are associated with the extensive native grasslands and winter wheat fields of the western counties. Grasshopper sparrow abundance coincides both with the western biogeographic origin of the bird, and also with the presence of contiguous areas of native grasslands in the western counties.
Relative abundance of the bobolink is influenced by the wet meadow habitats occurring along the route of the Kearney BBS. Johnsgard (1979) showed the southern limit of the bobolink breeding range in North America to closely parallel the Platte River Valley in Nebraska. Western meadowlark abundance nearly doubles upon approaching the 100 degrees W. longitude line. Distribution and abundance of this meadowlark reflects biogeographic origin more so than habitat availability, although lower numbers occurred in the heavily farmed regions of the eastern counties.
Red-winged blackbird abundance appeared to be relatively stable throughout, reflecting the ubiquitous habitat use of this species. Common grackle abundance, on the other hand, was a reflection of the eastern origin of this species, and the slow colonization of western habitats which this species is exhibiting. Preferred nesting habitat and feeding sites for common grackles are more prevalent in the eastern counties. Orchard oriole numbers appear to be greater in the western counties which may reflect a southwestern rather than southeastern origin of the Platte River population. Northern oriole numbers are greatest approaching the zone of overlap between the Baltimore and Bullock's subspecies. Relative abundance of the two subspecies appears to be similar within their usual ranges.
No discernible abundance patterns can be determined for the American goldfinch, although numbers are largest adjacent to the Platte River. The preponderance of house sparrows in the east is a reflection of greater human populations there, which contribute to the availability of nest sites and food sources.
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