Field Experiments in Prey Selection by Resident Bald Eagles in the Breeding and Non-breeding Season
نویسندگان
چکیده
--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. EXPERIMENTO PARA DETERMINAR LA SELECCION DE PRESAS DURANTE LA •POCA REPRODUCTIVA Y NO REPRODUCTIVA POR PARTE DE INDMDUOS RESIDENTES DE HALIAEETUS LEUCOCEPHALUS Sinopsis.--Se diseft6 un experimento de campo en el cual se le ofrecian como presa, a individuos de •tguila calva (Haliaeetus leucocephalus), peces de diferente tamafio. E1 objetivo del experimento fue determinar silas •tguilas ten/an preferencia por una presa particular y siestas preferencias variaban entre la &poca reproductiva y la no-reproductiva. Se 11evaron a cabo 67 pruebas en cuatro parejas de •tguilas; 32 durante la &poca reproductiva y 35 durante la no-reproductiva. Las •tguilas eleccionaron a la presa de mayor tamafio en el 71.8% de las ocasiones durante la &poca reproductiva yen tan s61o el 34.2% de las ocasiones durante la &poca no-reproductiva. Las rapaces tomaron el pez m•ts pequefio en nfmeros similares en cada temporada (25% en la &poca reproductiva y 28.6% en la no-reproductiva). Durante la •poca no-reproductiva, las •tguilas no respondieron al tamafio del pez en el 37.1% de los casos, en contraste a un caso de no respuesta (3.1%) durante la •poca reproductiva. E1 tiempo de respuesta l pez result6 m•ts corto durante la &poca reproductiva. De igual manera se respondi6 m•ts fftpidamente, en cualquiera de las &pocas, cuando se seleccion6 la presa de mayor tamafio. Se concluye que las •tguilas discriman sobre el tamafio de la presa, y pueden alterar la seleccitn de presas en respuesta a la demanda energ&tica como consecuencia de la &poca reproductiva. Estos resultados on consistentes con la prediccitn de los modelos de forageo 6ptimo, y ofrece otras razones, adem•s de cambios en la disponibilidad
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