Northern Prairie Wildlife Research Center
The Central Plains States (South Dakota, Nebraska, Kansas, and Oklahoma) account for more than 1500 of the 8300+ bald eagles wintering in the 48 contiguous states (Lincer et al. 1979). Wintering concentrations of this species in the midcontinent region are associated primarily with river systems. Because of the endangered status of the bald eagle, there has been an increasing need to identify its winter habitat requirements. Several recent investigations have increased our knowledge of the winter ecology of this species (Edwards 1969; Vian 1971; Lish and Lewis 1975; Steenhof 1976; Hansen 1977; Griffin 1978).
Although the Platte and North Platte Rivers in Nebraska have been identified as an important wintering ground of the bald eagle (Shickley 1961; Vian and Bleise 1974), limited information has been available on population distribution and abundance, habitat use, and feeding ecology. Therefore, a major emphasis was placed on obtaining such information as part of present investigations in order to identify habitat requirements and to develop guidelines for management of bald eagle habitat on the study area.
Lands within the Platte River Valley also host numerous other raptors during the winter. Previous studies have described species composition and winter densities of birds of prey in portions of Nebraska (Mathisen and Mathisen 1968; Craighead and Craighead 1969; Lock 1972, 1974) but there has been a general lack of information concerning the distribution and abundance of raptors in relation to specific habitat types and areas within the Platte River Valley.
Sixteen species of raptors were observed on the study area from 1978-30 including the endangered bald eagle and the peregrine falcon. The number and species composition of raptor sightings by reach of river are shown in Appendix F. A total of 12 species were sighted along 265 km (164 miles) of survey routes that were censused semi-monthly and monthly during the 1978-79 and 1979-80 winters, respectively. The bald eagle was the most numerous species with peak wintering populations estimated to be 150 and 250 birds during the 1978-79 and 1979-80 winters, respectively. The other principal raptors, in decreasing order of abundance, were the red-tailed hawk, American kestrel, marsh hawk, rough-legged hawk, prairie falcon, ferruginous hawk, and golden eagle. Incidental sightings included four separate observations of peregrine falcons, three of merlins, and 10 of golden eagles. Three species of owls were observed during surveys: the great-horned owl, screech owl, and short-eared owl.
The density of raptors wintering in the Platte and North Platte River Valleys declined from 0.46 birds/km (0.73 birds/mile) of survey route in 1978-79 to 0.39 birds/km (0.63 birds/mile) in 1979-80. Densities of bald eagles, rough-legged hawks, and sharp-shinned hawks increased during the latter winter whereas densities of red-tailed hawks, kestrels, marsh hawks, prairie falcons, ferruginous hawks, and golden eagles declined. Species differences appear related, in part, to marked yearly variation of weather conditions. The 1978-79 winter was extremely cold with a substantially greater snow cover than existed during the following winter. Mild conditions during 1979-80 caused most species to disperse because of improved foraging conditions. The bald eagle population, however, increased by approximately 50% during the 1979-80 winter because river channel conditions were more favorable for foraging than during the previous winter.
Bald eagles were observed throughout the 327 km (203 miles) of river channel during the study. However, densities varied widely among reaches of river (Fig. 16). Based on evening counts of individuals returning to communal roosts, the largest concentrations were on Jeffrey's Island (roost F) near Lexington and at two sites below Kingsley Dam (roost B). The number of bald eagles per roost, characteristics of roost sites, and roost locations are given in Appendix G.
Raptors wintering in the Platte River Valley were most often associated with riparian woodlands 44% of the sightings were recorded in this habitat type (Table 7). Habitat use varied widely among species. Buteos, particularly the bald eagle and red-tailed hawk, tended to use shelterbelts and woodlots whereas the ferruginous hawk, prairie falcon, American kestrel, and marsh hawk frequented tilled fields and mowed haylands. Irrigation ditches, and grazed and ungrazed pastures each accounted for less than 6% of the raptor sightings.
Bald eagles typically resided in riparian habitat (Table 7). However, eagles ranged widely from the river channel during the 1978-79 winter, apparently in search of food.
Bald eagles have a diverse diet while wintering along the Platte and North Platte Rivers in Nebraska. Analyses of the 2858 regurgitated pellets collected at 11 communal night roosts and several feeding perches during the 1978-79 and 1979-80 winters indicated the diet included 23 species of birds and 22 species of mammals (Appendix H). The most frequently occurring foods, by major taxa, were: birds, 76.5%; mammals, 33.8%; fish, 10.8%; and other, 0.3%. The principal prey species (by percent occurrence) were: mallard, 37.0%; eastern cottontail, 9.1%; Canada goose, 8.0%; carp, 5.0%; and meadow vole, 2.6%.
Although search efforts were comparable during the 2 years and despite a 67% increase in bald eagle numbers, only 793 pellets were collected at roost sites in 1980 as compared to 1989 pellets collected in 1979. The lower rate of pellet formation during the second year appears due to differences in diet. General observations indicated that eagles preyed more heavily on fish during the 1979-80 winter. Fish are more digestible than birds and mammals so fewer pellets are produced (Lish 1973). When fish were observed in pellets, their presence was indicated only by a few scales amid either hair or feathers. Less than 3% of the pellets were composed solely of scales. Therefore, the level of fish occurrence in the diet is underestimated from pellet analyses, emphasizing the need to also consider field observations of foraging patterns.
Eagle food habits were strongly influenced by prey availability. Other vertebrates formed a relatively low proportion of the diet when riverine conditions caused fish to be readily accessible. However, when either ice cover or high flows prevented eagles from obtaining fish, alternative prey were taken. Waterfowl dominated the diet during the 1978-79 winter when only limited ice-free channel habitat remained and snow cover was present. Observations indicated many of the waterfowl eaten were scavenged from other raptors. Bald eagles were observed stealing waterfowl remains from other raptors (klepto-parasitism) on at least 12 occasions during the 1978-79 winter and only twice during the 1979-80 winter; the importance of waterfowl in the diet of bald eagles declined markedly during the latter winter. Bald eagles were observed pirating mallards from ferruginous hawks, golden eagles, and other bald eagles. Carrion also was an important component of the diet under severe conditions during the 1978-79 winter. Several reports were received of eagles feeding on dead cattle left in fields along the river during that winter whereas no reports were received during the winter of 1979-80. The close proximity of the River to I-80 results in substantial carrion being available to eagles as a result of mortality from motorized vehicles. Deer-losses are substantial and contribute to the winter diet of eagles. Bald eagles also took advantage of winter-killed fish which became available during the spring melt. Observations of klepto-parasitism ended in early spring when alternative food sources became available. Bald eagles were never observed capturing live prey other than fish but were observed carrying or feeding on freshly killed carcasses of mallards, Canada geese, and blacktail jackrabbits on several occasions.
Prey availability varied from west to east. The eastern cottontail, for example, was more abundant at easterly sites, and was more prevalent in the diet there. Conversely, other species such as pronghorn and mule deer were more abundant in the west and were identified in pellets only at western roosts.
Marked temporal shifts in diet followed changes in prey availability. During the 1979-80 winter, for example, pellet numbers at roost sites increased markedly during late February and March as eagles shifted from a predominantly fish diet to one dominated by birds and mammals. This change was associated with high water conditions which apparently impeded eagle foraging success on fish. Pellet numbers increased from a low of 22 between 29 January and 11 February to a peak of 281 during 11-24 March while flows in the Platte at Overton increased from 44-72 m3/sec to 71-88 m3/sec, respectively. Field observations of eagles pursuing birds and mammals increased during the period of high flows while observations of eagles pursuing fish diminished. Blacktail jackrabbits became a major food item in the diet when high flows impeded access to fish. Jackrabbits occurred in 21% of the pellets during the 1979-80 winter in comparison to <1% the previous year. Pellet data were complemented by regular observations of bald eagles in pursuit of jackrabbits in the Overton Area. Under improved foraging conditions during the 1979-80 winter, klepto-parasitism diminished greatly suggesting that it occurs primarily in times of stress when alternative prey are scarce.
The diet of eagles varied among the roosts (Fig. 17). Waterfowl formed a high proportion of the diet at those sites where numerous mallards and Canada geese were present, i.e., below Kingsley Dam and downstream from the warm-water return flows from the Tri-County Diversion Canal (sites E thru I). Mammals formed a greater proportion of the diet in areas where fish and waterfowl were less available.