ADAPTIVE VARIATION IN THE FORAGING BEHAVIOR OF GREY PLOVER PLUVIALIS-SQUATAROLA AND WHIMBREL NUMENIUS-PHAEOPUS

Between-site variation in the diet and foraging behaviour of a fixed-m ethod forager, the Grey Plover Pluvialis squatarola, and a versatile f orager, the Whimbrel Numenius phaeopus, was examined and compared at n ine tropical and south temperate sites. Grey Plover always foraged in a run-stop-search manner but changed the emphasis of components of thi s behaviour in response to prey type. Whimbrel foraged tactilely or vi sually and readily changed foraging speed and habitat. Both species at e a variety of prey species depending on their availability. The diet of Grey Plover was dominated by small prey, polychaetes and crabs, and Whimbrel ate mostly crabs, with Whimbrel being the more specialized i n prey choice. The broader diversity of prey types consumed by Grey Pl over was attributed to limitations imposed by obligate visual foraging , whereas the flexible foraging behaviour of Whimbrel allowed the latt er to concentrate their efforts on the most profitable prey. Grey Plov er appeared to have a density upper limit, determined by their stereot yped foraging behaviour. Whimbrel densities varied greatly in response to prey type and foraging method. Foraging effort could not be predic ted from measurements of instantaneous daytime energy intake rates, an d reasons for the lack of this relationship are discussed.