The Ecology of the Wash. II. Seasonal Variation in the Feeding Conditions of Wading Birds (Charadrii)

Abstract

This study forms part of an investigation to predict the effect of the construction of proposed fresh-water reservoirs on the wading birds that feed on the macro-invertebrates of the Wash [England]. It evaluates the possibility that the birds had difficulty in obtaining their food at times between Aug. and May when they were present in the greatest numbers. Studies of daylight feeding routine showed that in winter waders generally spent least time in the roosts at high water and fed for the greatest proportion of the time when the feeding grounds were exposed. This was most marked in the small species, knot (Calidris canutus) dunlin (C. alpina) and redshank (Tringa totanus), which spent over 95% of the available daylight hours feeding in winter. They fed much less intensively at other times, especially in autumn. Feeding at night was not studied in most species, but many oystercatchers (Haematopus ostralegus) stopped feeding at night in the one winter in which this could be investigated. Apart from the Cerastoderma edule taken by oystercatchers, prey biomass was highest in autumn or early winter and declined sharply by spring, especially in areas where it had been initially high. The weight of an animal of certain length declined during the winter in a number of prey species. The numbers of bivalve mollusks taken per minute by knots decreased as the mud temperature decreased from 8 to -1.degree. C, but the feeding rate of oystercatchers, curlews (Numenius arquata) and bar-tailed godwits (Limosa lapponica) on a variety of invertebrates was not affected by temperature between 10-2.degree. C. Birds of most species studied were more likely to be found dead in winter than in autumn and spring. The extent to which waders had difficulty in collecting their requirements in winter is discussed. Some components which determine seasonal changes in the overall feeding conditions of 1 important sp., the knot, are also identified.