The importance of food in the determination of the winter flock sites of the Shelduck

Abstract

The distribution of the Shelduck Tadorna tadorna in the estuary was related to the overall distribution of the main prey Hydrobia. The areas with the most dense prey were the favoured sites. The proportion of time spent feeding by birds varied between flock sites and between years. The bird feeding hours also varied considerably. Prey was not distributed equally between the flock sites. The maximum density of selected sizes of Hydrobia was 47,670/m^2 and the minimum 3470/m^2. Hydrobia accounted for over 90% of the combined prey except where there were relatively high numbers of Corophium and Nereis. The proportion of time spent feeding was related to the combined prey density but the correlation was, in reality, due to the density of Hydrobia. Prey density does not define the relative importance of sites. The role of disturbance is discussed. The disadvantage of spending a large proportion of time feeding is probably not important except in occasional severe conditions.