The ecology of the Wash. 2. Seasonal variation in the feeding conditions of wading birds (Charadrii).
Abstract
2. Studies of feeding routine during daylight 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, knots (Calidris canutus) dunlins (C. alpina) and redshanks (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 studied. Apart from the Cerastoderma edule taken by oystercatchers, prey biomass was greatest in autumn or early winter and declined sharply by spring, especially in areas where it had been high initially. The weight of an animal of certain length declined during the winter in a number of species of prey. The numbers of bivalve molluscs taken per minute by knots decreased as the mud temperature decreased from 8 deg to -1 deg 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 deg and 2 deg . Birds of most species studied were more likely to be found dead in winter than in autumn or spring. The extent to which waders had difficulty in collecting their requirements in winter is discussed and some components which among them determine seasonal changes in the overall feeding conditions of one important species, the knot, are identified.