Sit-and-wait for large prey: foraging strategy and prey choice of White-tailed Eagles

Abstract

Little is known about foraging strategy and prey choice in large raptor species and how they might change with age and season. Here, we present results about time allocation, foraging pattern and diet selection of adult territorial White-tailed Eagles Haliaeetus albicilla from northeastern Germany. To assess age-related differences, we also observed foraging behaviour in roaming juveniles. Eagles allocated most of their diurnal time to perching. Since perch-hunting was more efficient than flight-hunting, “sit-and-wait” for prey seems to be a low-cost, highly profitable foraging mode in eagles. A linear mixed model revealed that season significantly affected eagle foraging patterns. Success in prey capture decreased and duration of foraging flights increased considerably in winter. Eagle strike success varied significantly between different territories and increased with increasing habitat quality. Adults foraged more efficiently than juveniles, presumably because of their superior spatial knowledge and hunting skills. A use–availability design for prey selectivity indices judged by log-likelihood chi-square statistics indicated that eagles make choices, within both their primary prey fish and alternative prey waterfowl, consistent with predictions of optimality models. When prey was abundant, eagles preferred large over small fish and slow over agile waterfowl species. Thus, prey choice by eagles reflected a complex function of absolute availability, size and anti-predator behaviour of their prey. Our study demonstrates that large raptors such as White-tailed Eagles are generally energy maximisers and pursue a “sit-and-wait” hunting mode to capture profitable prey, and can modify their foraging strategy to cope with variations in weather conditions and food availability.