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

Buffer effects occur when sites vary in quality and fluctuations in populat ion size are mirrored by large changes in animal numbers in poor-quality si tes but only small changes in good-quality sites. Hence, the poor sites `bu ffer' the good sites(1,2,) a mechanism that can potentially drive populatio n 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 bree ding grounds (an indicator of reproductive success(3,4)). The Icelandic pop ulation of the black-tailed godwit, Limosa limosa islandica, wintering in B ritain 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 estuari es with low initial numbers, and godwits on these sites have lower prey-int ake 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.