Factors influencing foraging decisions in ruddy turnstones Arenaria interpres (L.)

Abstract

Animals must assimilate energy to survive and reproduce, but foraging conflicts with other demands on an animal's time. We know very little about how animals resolve these conflicts in natural settings. I studied foraging choices made by ruddy turnstones Arenaria interpres (L.) using rocky coastline in north-east England. In particular I explored how foraging decisions varied with resource quality, the predictability of patch appearance, and perceived predation risk while using alternative patches. This study includes the first quantitative investigation into the use of beach-cast wrack by shorebirds. Energy intake per unit time by foragers on supratidal habitats was much higher than on intertidal habitats. However, birds exclusively used intertidal habitats when these were exposed by the tide, and moved onto supratidal habitats only over the high-water period. Moreover, the number of birds feeding over a given high tide did not depend on supratidal food availability. These results suggested that there were costs to foraging supratidally. Were some foragers being forced to pay these costs because of low foraging efficiency, or did some accept the costs because of other associated benefits? The use of supratidal habitats appeared to incur elevated predation risk for foragers; they were situated in areas where raptors could approach a foraging flock relatively closely before being detected. Accordingly, vigilance was much higher than expected on supratidal habitats, and increased with distance from the water's edge. Birds that regularly fed supratidally tended to be males, older and higher-ranking, and had smaller, less patchy home ranges than birds that rarely fed supratidally. This suggests that some birds were paying the cost of elevated predation risk associated with supratidal feeding for the benefits of stable group membership and higher social status, while others minimised their need for supratidal feeding by spatially tracking the variation in intertidal habitat quality.