Does an optimally foraging oystercatcher obey the functional response?

Summary. (1) We describe an experimental test of the opti-mal diet model. An oystercatcher was offered bivalves Scro-bicularia plana which were buried in the substrate at differ-ent depths. All prey were of equal length and thus of equal energy content. Since handling time increased with depth, deep-lying prey were less profitable. (2) From the known increase of handling and search time with depth we were able to predict for each prey density which depth classes should be ignored by the bird so as to maximize its intake rate. (3) The observed epth selection and intake rate were as predicted for the lower prey densities. At high prey densities the observed number of prey taken rose above the predicted intake rate. We suggest that the bird started to ignore closed bivalves with long handling times. (4) The observed relationship between prey density and in-take rate could not be described by Holling's type-2 func-tional response, because the handling time and the rate of discovery were density-related. A multi-species func-tional response is necessary to describe the intake rate of an optimally feeding predator. The functional response, the predation rate of an individual predator elated to differences in prey density, has been much studied during the last decades. In the simple one-prey situation, the intake rate of the predator often rises decelerating to a plateau (type-2 functional response; Holl-ing 1959). The curve has been described by the disc equa-tion: N a