Does an optimally foraging oystercatcher obey the functional response?

(1) We describe an experimental test of the optimal diet model. An oystercatcher was offered bivalves Scrobicularia plana which were buried in the substrate at different 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 depth 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 intake rate could not be described by Holling's type-2 functional response, because the handling time and the rate of discovery were density-related. A multi-species functional response is necessary to describe the intake rate of an optimally feeding predator.