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


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.

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Figure 1: Rates of change of godwit numbers differ at different sites.
Figure 2: Godwit populations that are initially smaller grow more rapidly.
Figure 3: Godwits from increasing populations have lower prey-intake rates and arrive later in Iceland.


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We are grateful for the financial and technical support of NERC; the TMR programme of the EC through Sandgerdi Marine Institute, Iceland; and A. Gardarsson and the Icelandic Institute of Natural History. We are grateful to L. Norton, J. Smart, D. McNeill, R. Croger, G. Appleton and T. Turner for assistance in the field, and to I. Côté, J. Reynolds and A. Watkinson for comments on the manuscript. Survival data were generated from Farlington Ringing Group and Wash Wader Ringing Group colour-ringing schemes. The FRG scheme is partly funded by the Farlington Ringing Group, Hampshire Ornithological Society and Hampshire County Council. Wetland Bird Survey data were provided by the British Trust for Ornithology.

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Correspondence to Jennifer A. Gill.

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Gill, J., Norris, K., Potts, P. et al. The buffer effect and large-scale population regulation in migratory birds. Nature 412, 436–438 (2001).

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