| Summary: | The aim of this study, carried out on the Ythan estuary, Aberdeenshire, between October 1973 and June 1976, was to investigate the processes, if any, operating to regulate the numbers of wading birds with respect to food availability in winter. Data were collected on the numbers and distribution of all waders using the estuary throughout the year and detailed observations were made on the feeding, dispersion and aggressive behaviour of Redshank. It was found that the distribution of several species of waders on the estuary varied seasonally. Most of the Redshank were found in the top end of the estuary when numbers were low or when daylength was short, suggesting that this area was preferred. An extensive sampling programme for the invertebrate prey of Redshank failed to show any seasonal changes in the abundance of invertebrates which might account for the overriding influence of daylength on the distribution of Redshank. However, observations of feeding Redshank in several areas of the estuary showed that the calorific intake per minute, and per feeding movement, were greatest in the preferred area throughout the year. Corophium and Nereis were found to dominate the diet, both numerically and in terms of biomass, in all areas; however, it appeared that Nereis was the more important prey since, in the two areas of the estuary which were studied in the greatest detail, the variation in the calorific intake from Nereis explained a very large, percentage of the variation in total calorific intake. Observations of the dispersion and intra-specific aggression of Redshank suggested a mechanism whereby the density of birds feeding in winter might be regulated with respect to food availability. It is proposed that, through increased intra-specific aggression and the holding of feeding territories, Redshank spread out) over the estuary in winter causing some Redshank to feed in sub-optimal areas'). The possibility that such regulatory mechanisms are widespread amongst wading birds wintering in Britain is considered.
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