Apparent Absorption Efficiencies for Redshank (Tringa totanus L.) and Oystercatcher (Haematopus ostralegus L.): Implications for the Predictions of Optimal Foraging Models

Apparent absorption efficiencies of dry mass for redshank (Tringa totanus) feeding on blowfly (Calliphora sp.) larvae and pupae, sardines (Sardina pilchardus), and laver spire snails (Hydrobia ulvae) and for oystercatcher (Haematopus ostralegus) feeding on the flesh of mussels (Mytilus edulis) varied from 27% to 88%. Prey with high indigestible ash-equivalent contents had lower apparent absorption efficiencies of dry mass and also lower apparent absorption efficiencies of ash-free dry mass. These relationships suggest that several superficially energetically inefficient foraging behaviors performed by the predators-for example, washing prey items and partial prey consumption-may maximize net energy gain. The relationships have wider implications for foraging models that predict partial prey consumption and optimal diet. With respect to partial consumption of prey, the relationships, in combination with a monotonic increase in ash-equivalent content during the time spent eating a prey item, would lead the predator to leave the prey earlier and to leave more prey than previously predicted. In combination with variations in ash-equivalent content between prey types, the relationships may also lead to frequency-dependent shifts in prey preference.