Northern Prairie Wildlife Research Center
Human and agricultural development in the Great Plains has affected migratory birds in two general phases. The first phase occurred before the turn of the century when market hunting was in its heyday and cropland was being molded out of the prairie. During this time much of the feeding and nesting habitat for migratory birds was lost. Some populations of migratory birds were reduced directly as a result of uncontrolled hunting. In the second phase of development, Platte River flows were regulated for irrigation and power generation.
Although water development began as early as the 1860's (direct diversion for irrigation), the primary impact on Platte River flows was not felt until the development of large storage reservoirs on the river between 1909 and 1940. Water diversion and storage has altered the historic streamflow of the Platte and resulted in the development of forest and shrub communities where once there had been an unvegetated, wide river channel of shifting alluvial sand. The development of forest vegetation in the Big Bend reach of the Platte (especially between Brady and Overton) has accelerated since the late 1930's. Kingsley Dam and Lake McConaughy in Nebraska, and other reservoirs on the North Platte River upstream from Lake McConaughy, have been the major factor responsible for alteration of the flow regime in the Platte since the 1930's and consequently for the development of the wooded vegetation on the floodplain. Following regulation of the Platte, the breeding birds that were adapted to unvegetated sandy river channel, standing water sloughs, marshes, and wet meadows were forced into the few remaining areas of prairie-riverine habitat.
Changes in the distribution of sandhill crane populations on the Platte River since the 1930's are an excellent example of how a species has been forced to use alternative riverine habitat. The reach of the Platte between Cozad and Lexington served as one of the largest roost site complexes for sandhill cranes in the 1930's and 1940's. As a result of forest encroachment, sandhill cranes have now completely abandoned this stretch of the Platte River. The cranes have gradually shifted their distribution to more open areas of the channel farther east. The river channel near Grand Island is now one of the most important roost sites for sandhill cranes on the Platte River. This area was sparsely used 20 years ago when the channel farther west provided suitable roosting habitat. Currently there is no habitat suitable for sandhill crane roosting east of Grand Island.
Because most bird species require a complex of habitat types for feeding, nesting, and roosting, their responses reflect the net effect of habitat changes on particular species. Changes in the availability of habitat, and the effects of these changes on survivorship were considered in determining the response for each population. Of the 233 species of migratory birds currently recorded in the Platte River Valley, 45% (104 species) have benefited by habitat changes, while 42% (98 species) have declined, and 13% (31 species) have not changed noticeably (Table 4). The impacts of habitat alteration on the migratory species that currently nest in the Platte River Valley have generally been less severe than the impacts on migrant bird populations.
The most vulnerable species in the valley (i.e., endangered or threatened) have generally been negatively affected by habitat changes. In an evaluation of the 28 most vulnerable species, 18 species (64%) have declined as a result of habitat changes, while only 10 species (36%) have benefited. Of the 4 endangered and threatened species present along the Platte River, all but the bald eagle have been negatively affected by habitat changes.
Species adapted to woodland and shrub communities have been the primary beneficiaries of the change in habitat. Most of these species have expanded their populations in response to the increasing woody vegetation on the Platte River floodplain and around farmsteads. At the time of settlement there were few woodlands along the Platte River or on the adjacent prairies. Species adapted to woodland communities were not common or abundant before settlement. Woodland species have gradually invaded the Platte River Valley from forests to the east and from the Rocky Mountains to the west. Although woodland species have been increasing in response to the changing environment, native grassland species have been declining. As a group, species associated with native lowland grasslands have undergone the most substantial population declines (U.S. Fish and Wildlife Service 1981). Species adapted to unvegetated alluvial sandbars have also suffered. The encroachment of woody vegetation on Platte River sandbars has reduced roosting and nesting habitat for sandhill cranes, whooping cranes, least terns, ducks and geese.
Ring-necked pheasants and some waterfowl have benefited by adapting their diets to feeding on waste grain, generally corn. Because of the abundance of waste corn in the valley, some flocks of waterfowl now winter along the Platte River instead of migrating farther south as they traditionally have done.
Many of the increasing woodland- and shrub-adapted migratory birds are common or abundant, and have ubiquitous distributions. Most of the species on the decline, however, are uncommon, species with localized distributions. For example, the great horned owl, northern flicker, and Bell's vireo are woodland species which are widely distributed throughout the midwest, as well as along the Platte River. Grasshopper sparrows, upland sandpipers, and dickcissels, however, are grassland species with relatively limited distributions in the Great Plains. Woodland and shrub communities are widespread throughout the midwest, while grassland communities are almost universally being destroyed.
Grassland habitat losses have been considerable throughout the Great Plains. These losses have been most significant in the area bounded by Texas, Illinois, Saskatchewan, and Colorado. To the east of central Nebraska, grassland losses have been considerably greater than those along the Platte River. In the more arid region to the west, grassland losses have been less severe. Because grasslands are being lost at an accelerated rate on the Great Plains, bird species adapted to this habitat type are in greater peril than those adapted to woodlands. The unique character of the prairie habitat surrounding the Platte River before settlement makes the loss of grasslands in the valley particularly devastating to breeding birds. The lush wetland bottomlands and prairies surrounding the broad, shallow channel of the historic Platte, provided an ideal, centrally located oasis for migratory birds on their route to northern breeding grounds. No other place in the flyway provided such a complex of grassland, wetland, and sandy riverbed habitat.
Much of the riverine habitat for migratory birds along the Platte River today is in a transitional stage of development from an open, unvegetated, sandy river channel, to a forest community. The gradual development of wooded vegetation on the Platte River floodplain has resulted in a mixture of tree and shrub communities at various intermediate stages of growth and species compositions. The structural diversity provided by these successional stages has resulted in a greater habitat diversity for migratory birds. We found 50 species of breeding birds using these early-growth lowland forests. Seven of the species recorded were not found outside of these woodlands. Bell's vireo, for example, is a species primarily confined to young willows on the river floodplain. The willow community provides less diverse habitat for migratory birds than mature cottonwood forests. Many of the species taking advantage of the transitional shrub and woodland habitats available today will eventually be displaced as the forest communities mature into a uniform habitat type.
The benefits of transitional and agricultural habitats are temporary. Bald eagles, for instance, have benefited from an increase in forest vegetation along the Platte River, particularly in areas where the forest borders open river channel. Bald eagles are primarily dependent on fish and waterfowl as a food source while they are in the Platte River Valley. Without protection of instream flows to maintain a fishery, and open river channel to provide roost sites for waterfowl, the food source for bald eagles would be severely limited and there would be little available habitat for this species. A transitional habitat of 50% trees and 50% open river channel, for instance, would be much more valuable to bald eagles than a channel almost entirely covered by mature forest trees. Waterfowl and ring-necked pheasants, as mentioned previously, have benefited from agricultural production of corn. There are no guarantees, however, that corn will remain the principal crop in the Platte River Valley. A change in the economics of growing corn could immediately eliminate this food source for these species. Furthermore, cornfields provide only one component of the habitat complex required by waterfowl and species such as the ring-necked pheasant. Corn is a food source for a relatively small portion of the year (winter months). Without nesting habitat, roosting habitat, and food sources other than corn, these species cannot reproduce and sustain their population. Cornfields alone cannot provide for all the habitat needs of species such as waterfowl and pheasants.
The widespread use of pesticides in croplands is also detrimental to waterfowl and other migratory birds that feed on waste grain. Pesticide residues in grain and the soils are ingested by birds feeding in croplands. Many of these pesticides are absorbed and accumulated in the fat tissues of the birds, and are eventually transferred and concentrated in species such as hawks and bald eagles, which are at a higher level in the food chain.