Winter ecology of bald eagles in southcentral Nebraska

Approximately 200 bald eagles wintered along a 370-km section of the Platte and North Platte rivers in Nebraska during the winters of 1978-79 and 1979-80. A preponderance of the wintering eagles were adults, with the adult:subadult ratio highest during the harsh winter of 1978-79. Nocturnal roosts were located primarily in tree plantings near the river, with mean tree age at the roosts ranging from 39 to 84 years. Bald eagles utilized a diverse prey base that included fish, birds, and mammals. Remains of 56 prey species were identified from pellets; 76% of pellets contained birds, 34% mammals, and 11% fish. Eagles foraged principally on fish when ice covered less than 80% of channels and water levels were moderate to low. Waterfowl and mammals dominated the diet when the river was almost entirely frozen or water levels were high. Mallards, eastern cottontails, and carp were the principal avian, mammalian, and piscine prey, respectively. Eagles traveled long distances from the river to feed on field-feeding waterfowl when alternate prey were not available. Fish were underrated in pellets because body components are more digestible than other major prey consumed.