The Role of Neophobia in Determining the Degree of Foraging Specialization in Some Migrant Warblers

Abstract

I conducted experiments to examine the psychological basis of foraging specialization in Dendroica warblers. The focus of the experiments was a comparison of wild-caught, immature bay-breasted and chestnut-sided warblers; I have studied the foraging ecology of these species during the winter in Panama. During the nonbreeding season, the bay-breasted warbler is a generalized forager compared to the chestnut-sided warbler. Individuals of four other species of Dendroica were tested in similar experiments to see how bay-breasted and chestnut-sided warblers compared with the genus as a whole. Consistent with field observations, chestnut-sided warblers obtained hidden mealworms from fewer unfamiliar objects than did bay-breasted warblers. They approached a similar number of objects, but were more timid and ambivalent. When offered a variety of "model microhabitats" with a conspicuous and familiar reward, chestnut-sided warblers were far more hesitant to approach and would often not feed from novel microhabitats. Individuals of all six species of Dendroica had consistent rankings in how rapidly they fed at the model microhabitats. This variation had a species-specific component, with bay-breasted warblers feeding most rapidly and chestnut-sided warblers feeding most hesitantly. These results suggest that the number of microhabitats visited by a warbler is the result of a dynamic interaction of attraction and fear. Individuals and species with greater aversion to novel foraging situations may be more specialized than less neophobic warblers. Shifts in a neophobia threshold may provide a relatively simple mechanism for varying foraging specialization among closely related species.