Competition and food selection: field tests of a theory.

A graphical model describing the optimal choices of two species competing for resources in two types of habitats is tested. Both species prefer taking resources from one of the habitat types, but one of the species (the dominant), by virtue of interference competition, can gain access to the better patch more readily than the other (the subordinate). The model begins with the result of single-species optimal foraging models: at low densities of birds, only the better patch type should be selected, but as density increases, both should be used. Interspecific competition should not lead to qualitatively different behaviours for the dominant species because the effects of the subordinate are weak. For the subordinate, however, there is a 3rd class of behaviours: under the pressure of high densities of the dominant, the subordinate may totally avoid the better patch and use only the poorer. The validity of the model's predictions was tested using three species of hummingirds: blue-throated Lampornis clemenciae, Rivoli's Eugenes fulgens and black-chinned Archilochus alexandri coming to feeders.-from Authors