Patterns of winter field use by Starlings Sturnus vulgaris: how important is prey depletion?

Abstract

From a knowledge of Ideal Free Distribution theory1 we might be able to predict temporal patterns of habitat use by Starlings Sturnus vulgaris on the basis of changes in resource availability. We measured depletion of soil invertebrates by Starlings by studying the foraging behaviour of individual Starlings held in enclosures. After one hour of foraging by these captive birds, there was a significant decline in the availability of prey. To investigate whether such prey depletion may influence the movements of wild birds between feeding areas we conducted all-day observations of an overwintering flock in the same habitat. Departure of Starlings from feeding areas could not be predicted on the basis of previous residence times in those areas, and there was no correlation between field residence times and subsequent return times. However, the flock was more likely to move to a nearby site when moving between feeding areas. We suggest that the level of foraging pressure created experimentally exceeded that likely to occur in a natural situation. Although Starlings may attempt to use fields according to habitat quality, as measured by food availability, their patterns of habitat use cannot be predicted on this basis.