The Ecology of the Wash. III. Density-Related Behaviour and the Possible Effects of a Loss of Feeding Grounds on Wading Birds (Charadrii)

Abstract

This paper describes the responses of 2 spp. of wading birds, the knot (Calidris canutus) and oystercatcher (Haematopus ostralegus) to their own density and to that of their prey and measures the impact of these and other wading birds on their food during the winter. With small numbers present, knots and oystercatchers fed in limited parts of the feeding grounds. The proportion found in the less preferred areas increased as the total numbers of birds present increased, suggesting that the presence of birds on preferred areas deterred others from feeding there. More aggressive encounters over food items and feeding sites occurred as density increased, particularly in winter. The evidence that aggressive encounters over food items reduced the feeding rate (numbers of items taken per min) of the participants was equivocal, but other forms of interference in feeding may have occurred in some species. Feeding rate of oystercatchers on cockles increased rapidly as the numerical density of the cockles increased up to approximately 250 m-2 but varied little thereafter. The biomass ingested per min (ingestion rate) increased as the biomass density of the cockles increased over a wide range of values, and oystercatchers were most numerous where food abundance was highest. Part of the spatial varition in the biomass of cockles could be attributed to large differences between areas in the weight of cockles of a particular shell length. It was estimated that wading birds took between 14 and 43% of their prey during the winter in the main feeding areas, although the proportion taken elsewhere was less. The results are used to evaluate the possible effects on wading birds of building a freshwater reservoir on the shore. The consequences of an increase in bird density in those preferred feeding areas that remain, an increase in the use of less preferred areas and a change in prey size preference on the ability of birds to obtain their daily food requirements in winter are discussed. [This study was conducted in England.].