Letters to Nature

Nature 412, 816-819 (23 August 2001) | doi:10.1038/35090566; Received 6 June 2001; Accepted 27 June 2001

Evidence of intra-specific competition for food in a pelagic seabird

S. Lewis1,2, T. N. Sherratt1, K. C. Hamer1 & S. Wanless2

  1. Department of Biological Sciences, University of Durham, South Road, Durham DH1 3LE, United Kingdom
  2. Centre for Ecology and Hydrology, Banchory Research Station, Hill of Brathens, Aberdeenshire AB31 4BW, United Kingdom

Correspondence to: T. N. Sherratt1 Correspondence and requests for materials should be addressed to T.N.S. (e-mail: Email: T.N.Sherratt@durham.ac.uk) or S.W. (e-mail: Email: swan1@ceh.ac.uk).

The factors affecting the population dynamics of seabirds have long intrigued biologists1, 2, 3, 4, 5. Current data suggest that density-dependent depletion of prey during the breeding season may regulate population size6, 7, 8, 9. However, much of the evidence for this has been circumstantial, and the underlying mechanisms are unclear5, 10. Here, we show that the per capita population growth rates of northern gannet Morus bassanus at colonies in Britain and Ireland have declined with increasing population size. Furthermore, direct observations reveal that the mean foraging trip duration of breeding gannets is positively correlated with colony size, both among colonies of different sizes in the same year, and within colonies as they change in size. To understand this phenomenon, we have developed a model which demonstrates that disturbance of fish alone can readily generate conditions under which gannets at larger colonies have to travel further to obtain food.