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Field experiments in prey selection by resident Bald Eagles in the breeding and non-breeding season



Field experiments in prey selection by resident Bald Eagles in the breeding and non-breeding season



Journal of Field Ornithology 65(4): 441-446



A field experiment, offering a choice of two prey fish of unequal sizes to foraging Bald Eagles (Haliaeetus leucocephalus), was devised. The objective of the field experiment was to determine if eagles preferred one prey item over another, and if this preference varied between breeding and non-breeding seasons. A total of 67 trials were conducted on four nesting pairs of eagles, 32 trials in the breeding season and 35 in the non-breeding season. Eagles selected the large fish 71.8% of the time during the breeding season, as opposed to 34.2% in the non-breeding season. Eagles took the small fish in about equal numbers in each season (25.0% breeding, 28.6% non-breeding). Eagles failed to respond to either fish size 37.1% of the time during the non-breeding season, however, as opposed to one instance of no response (3.1%) in the breeding season. Mean response time was generally shorter in the breeding season than in the non-breeding season, and eagles responded more quickly when they took the large fish irrespective of season. It is concluded that eagles discriminate between large and small prey items, and may alter their prey selection based upon increased energetic requirements of the breeding season. These results were consistent with predictions of optimal foraging models and suggest reasons, in addition to changing prey availability, why food habits of Bald Eagles vary between the breeding and non-breeding season.

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