MEPS 338:295-305 (2007)  -  doi:10.3354/meps338295

Annual variation in diets, feeding locations and foraging behaviour of gannets in the North Sea: flexibility, consistency and constraint

K. C. Hamer1,*, E. M. Humphreys1,2, S. Garthe3, J. Hennicke4, G. Peters5, D. Grémillet6, R. A. Phillips7, M. P. Harris8, S. Wanless8

1Earth Biosphere Institute and Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK
2British Trust for Ornithology Scotland, School of Biological and Environmental Sciences, Stirling University, Stirling FK9 4LA, UK
3Centre for Research and Technology Westkuste, University of Kiel, 25761 Büsum, Germany
4Institute of Zoology, University of Hamburg, 20146 Hamburg, Germany
5Earth & Ocean Technologies, Krummbogen 32, 24113 Kiel, Germany
6Percy FitzPatrick Institute, University of Cape Town, Rondebosch 7701, South Africa
7NERC British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, UK
8NERC Centre for Ecology and Hydrology, Banchory Research Station, Hill of Brathens, Aberdeenshire AB31 4BY, UK

ABSTRACT: Many seabirds nesting in areas bordering the North Sea have recently experienced large annual variation in breeding success, including reproductive failures in some cases. In contrast, the breeding success of northern gannets Morus bassanus has remained remarkably stable. The present study examines data from the large gannet colony at the Bass Rock (southeast Scotland) across 3 years, to assess the extent to which such stability may reflect both flexibility and consistency in diets and foraging behaviour. Adults exhibited great flexibility both in the species and sizes of prey consumed and in foraging trip durations, ranges and total distances travelled. They also showed a high degree of consistency in bearings of foraging trips and in behaviour at sea; the sinuosity of foraging tracks and average speed of travel was very similar each year and birds in all years spent about half their time at sea in flight. Adults returned to the nest at higher speeds from more distant foraging locations up to ca. 300 km from the colony, but speeds decreased for the farthest destinations (>ca. 400 km). Moreover, the relationship between trip duration and distances travelled at sea was asymptotic beyond ca. 60 h. These non-linear relationships probably reflected constraints on energy expenditure during flight. As a result, nest attendance was low in years with long average trip durations and chicks were left unattended and vulnerable to attack by conspecifics. These data suggest that while adults have so far been able to maintain high reproductive success in years of low prey availability, they may not be able to do so in future years if providing sufficient food for chicks entails any further increases in trip duration or foraging effort.

KEY WORDS: Morus bassanus · Wildlife telemetry · Geolocation · Home range · Optimal foraging

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