The ecology of the Wash. 3. Density-related behaviour and the possible effects of a loss of feeding grounds on wading birds (Charadrii).
Abstract
3. Responses of knots (Calidris canutus) and oystercatchers (Haematopus ostralegus) to their own density and to that of their prey, and the impact of these and other wading birds on their food during the winter are discussed. 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 increased, suggesting that the presence of birds on preferred areas deterred others from feeding there. There were more aggressive encounters over food and feeding sites 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 there may have been other forms of interference in feeding in some species.Feeding rate of oystercatchers on cockles increased rapidly as the numerical density of the cockles increased up to about 250/m2 but varied little thereafter. However, 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 was most abundant. Part of the spatial variation in the biomass of cockles could be attributed to large differences among 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 possible effects of building a freshwater reservoir on the shore are discussed. In particular, 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. An appendix reports energy consumption by redstarts and oystercatchers. The calculation of intake is described. For the redstarts daily intake was about 97 kcal and for oystercatchers 296 kcal, 4.2 and 5.8 times their estimated basal metabolic rates.