Letters to Nature

Nature 412, 436-438 (26 July 2001) | doi:10.1038/35086568; Received 27 March 2001; Accepted 1 June 2001

The buffer effect and large-scale population regulation in migratory birds

Jennifer A. Gill1, Ken Norris2, Peter M. Potts3, Tómas Grétar Gunnarsson4,5, Philip W. Atkinson6 & William J. Sutherland5

  1. Tyndall Centre, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK
  2. School of Animal & Microbial Sciences, University of Reading, Whiteknights, PO Box 228, Reading RG6 6AJ, UK
  3. Solent Court Cottage, Chilling Lane, Warsash, Southampton SO3 9HF, UK
  4. Institute of Biology, University of Iceland, Grensásvegur 12, IS 108 Reykjavík, Iceland
  5. Centre for Ecology, Evolution and Conservation, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK
  6. British Trust for Ornithology, The Nunnery, Thetford IP24 2PU, UK

Correspondence to: Jennifer A. Gill1 Correspondence and requests for materials should be addressed to J.A.G. (e-mail: Email: j.gill@uea ac.uk).

Buffer effects occur when sites vary in quality and fluctuations in population size are mirrored by large changes in animal numbers in poor-quality sites but only small changes in good-quality sites. Hence, the poor sites 'buffer' the good sites1, 2, a mechanism that can potentially drive population regulation if there are demographic costs of inhabiting poor sites. Here we show that for a migratory bird this process can apply on a country-wide scale with consequences for both survival and timing of arrival on the breeding grounds (an indicator of reproductive success3, 4). The Icelandic population of the black-tailed godwit, Limosa limosa islandica, wintering in Britain has increased fourfold since the 1970s (ref. 5) but rates of change within individual estuaries have varied from zero to sixfold increases. In accordance with the buffer effect, rates of increase are greater on estuaries with low initial numbers, and godwits on these sites have lower prey-intake rates, lower survival rates and arrive later in Iceland than godwits on sites with stable populations. The buffer effect can therefore be a major process influencing large-scale population regulation of migratory species.