Aspects of the ecology and behaviour of ringed and grey plovers Charadrius hiaticula and Pluvialis squatarola
Both plovers were studied at Lindisfame, Northumberland, during three non-breeding seasons; and the Ringed Plover while breeding at Lindisfarne and in NE Greenland. In winter both species fed principally on intertidal polychaetes, but prey differed in different sites. Food requirements in winter could not be met during daylight. Prey capture rate at night was lower than by day. Plovers carry large fat reserves in winter probably to buffer the effects of cold, windy weather, which reduces their feeding rates more than those of sandpipers. Plovers search for prey apparently by visual means (without the tactile element used by other waders). At the sand surface intertidal invertebrates increased their activity (which enabled plovers to detect them) with increasing temperature; also in some cases with foot- vibration by Ringed Plovers. With an increase in prey availability, the birds increased their capture rate first and then their selectivity of prey sizes and species. A simple model is developed to describe the foraging behaviour and its modifications in relation to prey availability and distribution. The presence of other shorebirds also affected prey capture rate. Nocturnal foraging is discussed in relation to visual foraging and invertebrate activity. An attempt is made to predict the minimum densities of prey required for energy balance in various conditions and to compare visual and tactile foraging strategies of plovers and sandpipers. Growth rates of Ringed Plover chicks were similar in all areas and diurnal rhythm in activity more marked in the Arctic. The development of foraging from hatching to first winter is described in comparison to that of adults and survival rates for various age-groups assessed. Food supply does not appear to limit growth or chick survival but feeding ability may become important in autumn.