DO OYSTERCATCHERS SELECT THE MOST PROFITABLE COCKLES

The profitability (energy content per second o f handling tim e) o f cockles, Cerastoderma edule, to oystercatchers, H aematopus ostralegus, increases w ith cockle size. In accordance w ith the predictions o f optim al foraging theory, oystercatchers selected the m ore profitable (i.e. larger) cockles. The percentage abandoned at the different sites increased with the m ean size taken, suggesting th a t attacks on large cockles were less likely to be successful. Oystercatchers use som e visual cues to find prey during the day, b u t use only tactile cues at night. They take smaller prey a t night, with a flesh content 25 % less than those taken during the day. LZsge A predator faces an array o f prey differing in energy content and handling times. T h e p ro ­ fitability of a prey type can be expressed in terms o f the energy gained per second spent handling the prey (M acA rthur & P ianka 1966; Charnov 1976). A m ajor assum ption o f optim al foraging theory is th a t predators can distinguish between prey of differing profitabilities and select the m ore profitable ones. In this pap er I describe a test o f this assum ption. • -