Northern Prairie Wildlife Research Center
The sandhills region developed from wind-deposited sand after the close of the last glacial epoch. Topography consists of rolling steep-sided slopes with level to slightly sloping valleys between the sand dunes. Soils are very sandy, well-drained, and vary in texture from loamy fine sand to fine sand. Within our study area, the sandhill prairie occurs generally north of the Platte and North Platte rivers in Lincoln, Keith, and Garden counties (Fig. 2). Small outliers of this community exist as scattered remnants in Kearney, Phelps, Buffalo, Hall, and Dawson counties.
Principal grasses of the sandhills prairie include sand bluestem (scientific names of plants are given in the Appendix), little bluestem prairie sandreed, switchgrass, sand lovegrass, blue grama, and needle-and-thread. Typical forbs are silky prairie clover, hoary puccoon, annual eriogonum, prairie coneflower, stiff sunflower, prairie sandwort, and little pricklypear. Leadplant, New Jersey tea, wild rose, western snowberry, and soapweed yucca are the principal shrubs of this community. Sandhill prairie was the most extensive habitat in the study area primarily because of the largely unaltered Sandhills.
Fifty-one bird species were recorded in this habitat, and 31 were considered to be breeding. Mean density of breeding birds was 110.7 pairs per km2. Grasshopper sparrow, western meadowlark, and lark bunting were the most numerous species, making up 45.8% of the breeding population. Sixteen species reached their highest density in sandhill prairie: Swainson's hawk, greater prairie-chicken, sharp-tailed grouse, long-billed curlew, great horned owl, burrowing owl, common nighthawk, horned lark, black-billed magpie, rock wren, and savannah, grasshopper, vesper, lark, Brewer's, and Cassin's sparrow. Six species were restricted in this habitat (Table 1).
The mixed prairie community exists on nearly level terrain both north and south of the Platte River in Hall, Buffalo, Dawson, Lincoln, Gosper, Phelps, Kearney and Adams counties (Fig. 2). Because of its topography, most of the mixed prairie has been converted to agricultural production. Soils are deep silty loams derived from loess deposits.
Principal grasses of this prairie community include big bluestem, little bluestem, sideoats grama, switchgrass, Indian grass, blue grama, western wheatgrass, junegrass, and needle-and-thread. Common forbs include scurfpea silverleaf scurfpea, dotted gayfeather, false boneset, Missouri goldenrod, purple prairie-clover, panicled aster, scarlet globemallow and eastern pricklypear. Typical shrubs include lead plant, western snowberry, white coralberry, smooth sumac, and soapweed yucca.
Characteristic breeding birds of the mixed prairie community include red-tailed hawk, Swainson's hawk, killdeer, upland sandpiper, burrowing owl, common nighthawk, horned lark, American crow, grasshopper sparrow, and western meadowlark.
The shortgrass prairie community exists in the western region of the study area where average precipitation varies from 33 to 43 cm per year. Topography consists of a nearly level to gently sloping plain, dissected in numerous areas by the intermittent streams. Soils are well drained and are mainly loams or silt loams. Because of the flat topography and productive soil, much of the shortgrass prairie has been converted to agricultural production. About 73% of Deuel County, which lies entirely within this community, has been developed for cropland. The shortgrass prairie occurs generally south of the North Platte River in Keith, Garden and Deuel counties (Fig. 2).
Common grasses of this community include prairie sandreed, western wheatgrass, blue grama, threadleaf sedge, buffalograss, little bluestem, sand dropseed, green needlegrass, and Indian ricegrass. Forbs include scurfpea, silverleaf scurfpea, dotted gayfeather, Platte lupine, and broom snakeweed. The shrub layer is generally characterized by rabbitbrush, silver sage, and soapweed yucca.
Characteristic breeding birds of the shortgrass prairie community include Swainson's hawk, long-billed curlew, burrowing owl, common nighthawk, loggerhead shrike, horned lark, black-billed magpie, rock wren, western meadowlark, grasshopper sparrow, lark sparrow, lark bunting, Brewer's sparrow, and Cassin's sparrow. The latter two sparrows occur almost exclusively on shortgrass prairie dominated by rabbitbrush.
The sandsage prairie consists primarily of rolling hilly sand dunes stabilized by grass-like vegetation generally south of the South Platte River in Lincoln County (Fig. 2). The soils are well-drained sands derived from wind-deposited sand sediments.
Principal grasses of this community include sand bluestem, little bluestem, prairie sandreed, needle-and-thread, blue grama, switchgrass, hairy grama, and purple three-awn. Characteristic forbs include stiff sunflower, prairie spiderwort, annual eriogonum, hairy goldaster, dotted gayfeather, scurfpea and eastern pricklypear. Sand sagebrush is the primary shrub species associated with the sandsage prairie.
Characteristic breeding birds include Swainson's hawk, greater prairie-chicken, killdeer, upland sandpiper, long-billed curlew, mourning dove, common nighthawk, horned lark, loggerhead shrike, lark sparrow, lark bunting, grasshopper sparrow, western meadowlark, and brown-headed cowbird.
Development of riparian forest vegetation in the study area closely parallels the reduction of river flow levels brought on by the construction of water diversion projects in the upstream reaches of the valley (Frith 1974). Currier (1982) reported that most stands of riparian forest in the Platte and North Platte rivers appeared to be of uniform age, suggesting that widespread forest development occurred after changes in hydrological conditions. Reductions in river levels have impeded the scouring action of ice and the shifting of stream sediments.
These actions have greatly enhanced the development of forest vegetation. Currier (1982) presented an excellent description of the developmental history and dynamics of Platte River riparian vegetation.
Currier (1982) identified nine distinct forest vegetation types in the area from Lake McConaughy to Merrick County. These types vary in tree species, composition, soils, biogeographic distribution, and the mixture of shrub species. The cottonwood/cedar vegetation type is quite prevalent throughout the valley. Cottonwood is the predominant overstory plant species, with red cedar and rough-leaved dogwood important in the shrub layer. Other important tree species in Platte River riparian forests include green ash, American elm, and diamond willow. Russian olive, river-bank grape, wild rose, false indigo, and coyote willow are important shrub species. Kentucky bluegrass, poison ivy, common ragweed, black medic, white sweetclover, false soloman's seal, water sedge, and Canada goldenrod are important ground layer species.
Breeding bird populations in riparian forest ranked second in abundance among habitats studied, although considerable variability was evident on several plots censused both years. Twenty-seven species reached their maximum density in this habitat type (Table 1). The most abundant species were house wren, mourning dove, American robin, and warbling vireo. Eight species occurred only in this habitat type: yellow-billed cuckoo, eastern wood-pewee, northern rough-winged swallow, white-breasted nuthatch, Bewick's wren, cedar waxwing, red-eyed vireo, lazuli bunting, and rufous-sided towhee.
Riparian habitats are important to many wildlife species, especially in regions with intensive agriculture where streamside habitats are being depleted rapidly (Stauffer and Best 1980). Breeding bird densities in Platte River riparian forest were quite similar to the 678 pairs per km2 reported by Zimmerman and Tatschl (1975) in mature lowland forest along the Missouri River in eastern Kansas. Fawver (1947) reported a breeding density of 356 per km2 from mature floodplain forest in Illinois. Karr (1968) however, reported 1,208 pairs of 32 species per km2 in mature lowland forest in Illinois. Stamp (1978) reported 1,690 pairs per km2 from riparian cottonwoods in Arizona, and Stauffer and Best (1980) found 1,265 pairs per km2 in Iowa.
Wooded River Channel Island
Islands that are raised above the river channel and on which the vegetation has stabilized by woody growth are typically dominated by shrub vegetation. The islands dominated by dense shrub growth have an open, sandy understory with scattered grasses and forbs. Although widespread throughout the study area, this community is most prevalent in Dawson, Gosper, Phelps, Buffalo, Kearney, Adams, Hall, Merrick, and Hamilton counties (Currier 1982).
Considerable variation exists in river channel island vegetation, primarily because of varying degrees of soil wetness. In general, this community is dominated by coyote willow, false indigo, eastern cottonwood, and diamond willow. Red-osier dogwood is locally abundant. Important understory species include common ragweed, fog fruit, prairie cordgrass, narrowleaf aster, Canada goldenrod, cocklebur, and Japanese brome. Downy brome, white sweet clover, and poison ivy are important understory species on islands where red-osier dogwood and false indigo are prevalent overstory plants.
Wooded river channel islands represent a small proportion of available habitats, but their significance was demonstrated by the high degree of use made by breeding birds. Forty-one species were recorded on river channel islands, and 39 were considered breeders. Mean density of breeding birds was 523 pairs per km2. Cliff swallow and common yellowthroat were the most numerous species, making up 28.2% of the population. Fourteen species reach their highest density on river channel islands: killdeer, piping plover, spotted sandpiper, willow flycatcher, western wood-pewee, bank swallow, barn swallow, cliff swallow, Bell's vireo, yellow warbler, common yellowthroat, Indigo bunting, American goldfinch, and field sparrow.
Rocky Mountain Forest
Small outliers of coniferous vegetation resembling that associated with the Pine Ridge region of Nebraska occur along the North Platte River in Keith and Garden counties. This habitat type occurs on moderately steep north-facing slopes and in adjacent canyons. Soils are generally shallow loams or silts formed from the underlying sandstones.
Principal plant species associated with the ground layer of this community include little bluestem, prairie sandreed, blue grama, plains muhly, needle-and-thread, sand bluestem, western wheatgrass, Canada wildrye, green needlegrass, and Kentucky bluegrass. Forbs include shell-leaf penstemon, broom snakeweed, prairie goldenpea, false boneset, scarlet globemallow, Missouri goldenrod, and fringed sage. Typical shrubs include silver sage, chokecherry, prickly rose, poison ivy, and soapweed yucca. Rocky Mountain juniper is the predominant tree species, but green ash and box elder occur occasionally.
Characteristic breeding birds include mourning dove, northern flicker, western kingbird, black-billed magpie, house wren, American robin, loggerhead shrike, yellow-breasted chat, chipping sparrow, field sparrow, lark sparrow, brown-headed cowbird, and American goldfinch.
Wetlands on the Sandhills developed as groundwater seepage areas in the valleys of wind-deposited sand dunes. Soils of the region are principally fine sands of various taxonomic series. sandhills wetlands within our study areas are slightly to moderately alkaline (McCarraher 1977). Agricultural development is less intense in this region; the principal land use is livestock grazing of the surrounding native grasslands.
Prairie wetlands occur within the rainwater basin region of Adams, Kearney,Phelps, and Gosper counties, and in the sandhills region of northern Lincoln and southern Garden counties. Wetlands of the Rainwater Basin occur as closed systems resulting in basins ranging from <1 to 390 ha. The wetlands formed on irregular loess deposits modified by wind action (Evans and Wolfe 1967). The Rainwater Basin is characterized by flat to gently rolling topography. Soils are quite fertile, but contain a high percentage of clay which makes them susceptible to both drought and flooding. Nearly 95% of the wetlands in the rainwater basin have been drained and converted to cropland. Emergent vegetation of Rainwater Basin wetlands is characterized by hybrid cattail, hardstem bulrush, American bulrush, and various smartweeds (Evans and Wolfe 1967). Vegetation of Sandhills wetlands is dominated by hardstem bulrush, American bulrush, and common reed. The principal submerged aquatic species include sago pondweed, muskgrass,coontail, and water milfoil.
Breeding bird populations in natural basin wetlands are characterized by waterfowl, rails and coots, and blackbirds. Twenty-one species were recorded in natural basin wetlands, and 18 species were considered breeders. Mean density of breeding pairs was 533 pairs per km2. Red-winged blackbirds, yellow-headed blackbirds, mallards and American coots were the most numerous breeding species, making up 54.1 percent of the population. Fifteen species reached their highest breeding density in natural basin wetlands: pied-billed grebe, green-backed heron, mallard, northern pintail, green-winged teal, blue-winged teal, northern shoveler, sora, American coot, Wilson's phalarope, marsh wren, yellow-headed blackbird, red-winged blackbird, swamp sparrow and song sparrow. Eleven of these species were restricted to natural basin wetlands.
The wet meadow community occurs along river channels and other low areas in open grasslands and forests (Currier 1982). Because of its origin within the river floodplain, topography is essentially flat. Soils are typically poorly drained silty clay to loamy fine sands derived from accumulation of organic matter on alluvial sediments. Wet meadows are distributed throughout the study area, but are more common in the east where they are associated with grasslands on Fort Farm Island, Shoemaker Island, and Mormon Island. The grasslands generally consist of grazed pastures with ribbons of wet meadow vegetation in depressions that follow the natural drainage patterns (Currier 1982).
Vegetation is dominated primarily by sedges including American bulrush, spikerush, fescue sedge, wooly sedge, woodland sedge, Mead's sedge, saw-beak sedge, and fox sedge. Reed canary grass is most important among the grasses. Principal forbs include fog fruit, fringed loosestrife, and lady's thumb. Wet soils and grazing by livestock generally reduce the shrub layer in this community; coyote willow and false indigo are the two most prevalent (Currier 1982).
Thirty-one species were recorded in wet meadows, and 27 of these were considered breeders. The mean density of breeding birds was 110 pairs per km2. Western meadowlark, grasshopper sparrow, and brown-headed cowbird were the most numerous species, making up 40.8 percent of the population. Five species reached their highest densities in wet meadows: ring-necked pheasant, upland sandpiper, bobolink, eastern meadowlark, and western meadowlark. The eastern meadowlark was the only species restricted to wet meadows.
Riverine wetland vegetation occurs primarily in areas of standing water behind dams and in isolated pools of water adjacent to the river channel (Currier 1982). Areas of riverine wetland vegetation occur most frequently along the North Platte River and in widely scattered reaches of the Platte River between Lexington and Grand Island. Hardstem bulrush, American bulrush, cattail, spikerush, water sedge, fog fruit, and coyote willow make up the predominant emergent vegetation of this community.
Characteristic breeding birds of riverine wetlands include pied-billed grebe, great blue heron, green-backed heron, wood duck, mallard, virginia rail, sora, killdeer, belted kingfisher, eastern phoebe, tree swallow, northern rough-winged swallow, bank swallow, cliff swallow, barn swallow, marsh wren, yellow warbler, common yellowthroat, red-winged blackbird, yellow-headed blackbird, and brown-headed cowbird.
Open River Channel Island
This community is made up of exposed sandy deposits within the river channels. Open River Channel Islands are usually exposed during the summer as river levels decline from increased evapotranspiration, low precipitation, and increased water demands for agricultural production. Higher river levels during spring result in scouring the islands and the removal of short-lived plant species (Currier 1982). Although well distributed throughout the Platte River Valley, the bulk of this community occurs in Hall County between Alda and Grand Island, and in Lincoln County between Hershey and North Platte.
Vegetation characteristic of open river channel islands is typically annuals and biennials that can become established quickly during dry periods. Dominant plant species of this community include lovegrass, various nutsedges, cocklebur, barnyard grass, and sand dropseed.
Breeding birds of this open community include piping plover, killdeer, spotted sandpiper, and least tern. The plover and tern are especially characteristic of open sandbars, and their range and breeding populations have apparently declined dramatically in response to increasing woody vegetation encroachment of the sandbars.
Long, narrow, belts of trees and shrubs occur at the periphery of agricultural fields and near farmsteads. Shelterbelt establishment was encouraged by the Timber Culture Act of 1873. The Act provided for the acquisition of an additional free section (252 ha) of land for each 4 or more ha of shelterbelt trees planted (Albertson and Weaver 1945). In 1979, shelterbelts occupied about 8,300 ha (0.3%) of the study area.
Cottonwood and red cedar are the predominant tree species planted in shelterbelts. Other frequently encountered tree species include Russian olive, green ash, American elm, slippery elm, red mulberry, box elder, silver maple, hackberry, Chinese elm, and Siberian elm. The ground layer is usually poorly developed, consisting of various grasses including Kentucky bluegrass and timothy.
Characteristic breeding birds of shelterbelts include mourning dove, red-headed woodpecker, northern flicker, western kingbird, eastern kingbird, blue jay, house wren, American robin, brown thrasher, European starling, yellow warbler, northern cardinal, common grackle, brown-headed cowbird, orchard oriole, and house sparrow.
Data on the estimated number of pairs per km2 for each habitat type indicate that largest breeding densities occurred in shelterbelts (Table 1). Populations were similar between years thus suggesting little year-to-year variation. Shelterbelts were used by 26 species, of which 22 were considered nesting species. Eleven species reached their highest density in shelterbelts: red-headed woodpecker, eastern kingbird, and northern oriole were most abundant.
Yahner (1980) reported considerably higher breeding densities in four mature Minnesota shelterbelts. His densities ranged from 3,440 to 9,800 pairs per km2. Common grackles and mourning doves were the most abundant breeding birds; greatest densities for those bird species were 8,419 and 709 pairs per km2, respectively (Yahner 1980). Ressell (1973 a,b,c,d) reported densities ranging from 699 to 2,137 pairs per km2 in mature shelterbelts in eastern North Dakota. Mourning dove, yellow warbler, and American robin were the three most abundant species. Species diversity in Ressell's area ranged from 4 to 14 species. H. A. Kantrud (pers. comm.) found that breeding bird densities in central North Dakota shelterbelts averaged 803 pairs per km2 and consisted of 33 species.
The total number of species occupying shelterbelt habitat during the nesting season was among the lowest of all habitats (Table 1). Species diversity and equitability in shelterbelts were among the highest of all habitat in the Platte River Valley. Species diversity of a habitat is determined by the number of species in the habitat and the number of individuals of each species that are present. Equitability is highest when all species in the sample are as nearly equal in population as is possible (Kricher 1972). Although the number of species of breeding birds occupying shelterbelts is relatively low, their densities indicate that minimum habitat requirements are being met for a number of species.
About 55% of the land area within our study area was devoted to some form of crops in 1969 (Bose 1977). The intensity of agricultural land use ranged from 14% in Garden County to 84% in Hamilton County (Table 2). Because much of the study area was originally native grassland. The changes wrought by agriculture have made dramatic inroads into avian populations and species diversity. Current agricultural practices of fencerow to fencerow farming, summer fallowing, fencerow removal and shelterbelt removal to facilitate expanded use of center-pivot irrigation systems have placed additional stresses on native bird communities.
Corn production is the predominant use of much of the land in the eastern half of the study area. Large areas of land in Buffalo and Dawson counties are devoted to alfalfa production. Winter wheat is grown extensively in the western counties of Lincoln, Keith, Garden and Deuel. The breeding avifauna of cropland is decidedly barren in comparison to natural or other man-made habitats. Among land uses, the fewest breeding bird species occupy cornfields. The horned lark is most characteristic of cornfields. This holds only until after emergence of the corn which greatly diminishes the suitability of cornfields to that bird species. Of the 17 major habitat types studied by Graber and Graber (1963) in Illinois, row crops (corn and soybeans) consistently supported the lowest mean nesting densities.
Breeding bird populations in corn fields were the third lowest among all habitats in the study area. Eighteen bird species were recorded in corn fields, of which only three (horned lark, killdeer, and western meadowlark) were considered breeders. The mean density of breeding birds was 63 pairs per km2. Of the three breeding species, the western meadowlark made up 47.8 percent of the population. No species reached its highest density in cornfields. Graber and Graber (1963) reported that the nesting avifauna of Illinois cornfields was dominated by killdeer, horned lark, and vesper sparrow. Mean breeding density in cornfields across Illinois ranged from 74 to 222 pairs/km2.
Twenty-eight species of birds occupied alfalfa fields in our study area, of which 13 species were considered nesting. The mean density was 101 pairs per km2. Dickcissel, red-winged blackbird, western meadowlark and brown-headed cowbird occurred in highest densities. No species reached its maximum breeding density in alfalfa fields. Graber and Graber (1963) found nine species nesting in Illinois alfalfa fields; red-winged blackbird and dickcissel were the two most numerous nesting species.
Eight species were recorded in domestic pastures. The mean density of breeding birds was 58 pairs per km2. Red-winged blackbird and western meadowlark occurred in greatest densities, making up 52.6 percent of the population. No species reached its maximum breeding density in domestic pastures.
The mean breeding density of 48 pairs per km2 in winter wheat fields was the lowest recorded in the study area. Thirteen of the fifteen species recorded in wheat fields were considered to be nesting. Lark bunting, western meadowlark and grasshopper sparrow were most abundant, making up 60 percent of the population. Only one species, the lark bunting (at 12 pairs/km2) reached its greatest population density in wheat fields. Nesting densities in southern Illinois wheat fields ranged from 25 to 74 pairs per km2 (Graber and Graber 1963). Dickcissel, meadowlark sp., and grasshopper sparrow were the three most numerous nesting birds in central Illinois wheat fields (Graber and Graber 1963).
Numerous small towns and cities within the study area provide a wide range of habitats available for breeding birds. Prominent among these are deciduous and coniferous ornamental plantings around residences, parks and cemeteries, industrial areas, grain elevators, building ledges, and landfills. Diverse food sources are provided in these settings, which accommodate the existence of numerous bird species. Graber and Graber (1963) stated that residential habitats in Illinois supported higher densities of breeding birds than any other habitat except ecotones.
Characteristic breeding birds of residential habitats include rock dove, mourning dove, common nighthawk, chimney swift, downy woodpecker, western kingbird, purple martin, barn swallow, blue jay, black-capped chickadee, white-breasted nuthatch, house wren, American robin, northern mockingbird, European starling, northern cardinal, rose-breasted grosbeak, chipping sparrow, common grackle, northern oriole, house finch, and house sparrow.
The estimated density of breeding pairs in residential habitats was 624 pairs per km2 (Table 1). Populations between years were quite variable, related primarily to wide fluctuations in the number of house sparrows in each plot. Residential habitats were used by 23 species, and eight species reached their maximum density here. The breeding birds of residential areas were dominated by four species, which accounted for 61.8% of the population: house sparrow, common grackle, chimney swift, European starling. Chimney swifts had the most stable population of the common breeding birds in residential areas. Several species of irregular occurrence in south-central Nebraska were found in residential habitats including pine siskin and house finch.
Dutch elm disease has spread throughout the Platte River Valley, and there was an abundance of dead branches and natural cavities, which provided numerous places for hole-nesting European starlings and house sparrows. The general lack of other hole-nesters including northern flicker, black-capped chickadee and white-breasted nuthatch in residential habitats was probably due to competitive exclusion by the more aggressive house sparrows and European starlings.
Our population estimate of 624 pairs per km2 is quite similar to estimates of 588 and 600 in Ohio (Claugus 1977, 1978), 610 in Manitoba (Erskine 1972) and 645 in New Jersey (Waal Melefyt 1977).