FAILURE OF SIMPLE OPTIMAL FORAGING MODELS TO PREDICT RESIDENCE TIME WHEN PATCH QUALITY IS UNCERTAIN

Blue jays (Cyanocitta cristata) were presented with a foraging situation in which half of the patches they encountered contained no prey and half contained a single prey item. Experimentally determined probability distributions controlled prey arrival times in those patches that contained prey. Patch residence in empty patches was studied during four experiments. In the first, prey arrival was exponentially distributed. Residence times increased with travel time as predicted by a rate-maximization model, but the bird stayed in empty patches much longer than predicted. During the second experiment, prey arrival was uniformly distributed. The jays again stayed longer than optimal, and patch residence times increased as travel time increased, although the residence time that maximized rate of intake was independent of travel time under these conditions. In the third experiment, exponential and uniform patches were randomly intermixed. The jays showed larger travel-time effects in the exponential than in the uniform patch. However, the travel-time effect in the uniform patch was contrary to rate-maximization predictions, and the birds again overstayed in both patch types. In the fourth experiment, prefeeding at the start of each foraging bout slightly increased overstaying rather than decreasing overstaying, as would be expected if overstaying were due to underestimating environmental quality. Consistent and dramatic overstaying and a travel-time effect under conditions where travel time has no effect on optimal residence times suggest that the rate-maximization approach does not apply to foraging problems involving patch uncertainty.