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Journal of Ornithology

, Volume 157, Issue 1, pp 165–178 | Cite as

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

  • Mirjam Nadjafzadeh
  • Heribert Hofer
  • Oliver Krone
Original Article

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.

Keywords

Dietary preferences Foraging strategy Haliaeetus albicilla Hunting mode Raptor conservation Time allocation 

Zusammenfassung

Sitzen-und-Warten auf große Beute: Nahrungssuchstrategie und Beutewahl des Seeadlers Über die Nahrungssuchstrategie und Beutewahl großer Greifvogelarten und wie diese sich mit dem Alter und der Jahreszeit verändern können, ist bisher wenig bekannt. Hier stellen wir Ergebnisse über die Zeitbudgets, Nahrungssuchmuster und Nahrungswahl adulter territorialer Seeadler Haliaeetus albicilla aus Norddeutschland vor. Um altersbedingte Unterschiede zu erfassen, untersuchten wir auch das Nahrungssuchverhalten von umherstreifenden Juvenilen. Seeadler wendeten ihre meiste Tageszeit für die Ansitzjagd auf. Da diese passive Nahrungssuche effizienter war als aktive Suchflüge, scheint „Sitzen-und-Warten“ auf Beute eine mit geringen Kosten und hohem Nutzen verbundene Jagdmethode bei Seeadlern zu sein. Ein lineares gemischtes Modell ergab, dass die Jahreszeit die Nahrungssuchmuster von Seeadlern signifikant beeinflusste. Der Erfolg beim Beutefang sank und die Dauer der Nahrungssuchflüge stieg beträchtlich im Winter. Der Beutefangerfolg der Seeadler variierte signifikant zwischen verschiedenen Territorien und nahm mit steigender Habitatqualität zu. Adulte waren bei der Nahrungssuche effizienter als Juvenile, vermutlich aufgrund ihrer besseren Raumkenntnis und Jagdfähigkeiten. Ein Nutzung-Verfügbarkeitsdesign für Nahrungswahlindizes, die mit log-likelihood Chi-Quadrat-Anpassungstests geprüft wurden, wies darauf hin, dass Seeadler selektieren, sowohl innerhalb ihrer Hauptbeute Fische als auch ihrer alternativen Beute Wasservögel, übereinstimmend mit Vorhersagen des Modells zur optimalen Nahrungswahl. Wenn ein reichliches Angebot an Beutetieren vorhanden war, präferierten die Seeadler große vor kleinen Fischen und langsame vor agilen Wasservogelarten. Demnach reflektierte die Beutewahl bei Seeadlern eine komplexe Funktion von der absoluten Verfügbarkeit, Größe und dem Feindabwehrverhalten ihrer Beutetiere. Unsere Studie demonstriert, dass große Greifvögel wie Seeadler generell Energiemaximierer sind und zum Fang von profitabler Beute eine „Sitzen-und-Warten“ Jagdmethode verfolgen, sie ihre Nahrungssuchstrategie jedoch modifizieren können, um sich an veränderte Wetterbedingungen und eingeschränkte Nahrungsverfügbarkeiten anzupassen.

Notes

Acknowledgments

We are grateful to the administration of the nature park Nossentiner/Schwinzer Heide and the Reepsholt-Stiftung for logistic support and accommodation. We thank W. Neubauer for data on waterfowl availability at the Krakower Obersee. We are indebted to several fishermen, forestry districts, hunters and landowners in the study area for collaboration and support, and to F. Scholz, J. Sulawa, A. Trinogga, and N. Kenntner for assistance and support. We thank the editors and anonymous reviewers for very helpful comments on a previous version of the manuscript. This study was funded by the Federal Ministry of Education and Research (BMBF, reference no. 0330720) and the Leibniz Institute for Zoo and Wildlife Research Berlin.

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Copyright information

© Dt. Ornithologen-Gesellschaft e.V. 2015

Authors and Affiliations

  • Mirjam Nadjafzadeh
    • 1
    • 2
  • Heribert Hofer
    • 1
    • 2
  • Oliver Krone
    • 1
  1. 1.Leibniz Institute for Zoo and Wildlife ResearchBerlinGermany
  2. 2.Freie Universität BerlinBerlinGermany

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