Seasonal Variation in Feeding Habits of Darwin's Ground Finches

Abstract

We investigated the effects of seasonal variation in food abundance on food partitioning among Darwin's ground finches of the genus Geospiza. Finch populations, the availability of seed of fruits, and finch feeding habits were measured during 2 visits to each of 4 study sites on 3 Galapagos islands. Our 1st visit was made near the end of the 1973 rains at a time of seasonal food abundance and the 2nd, 6—8 mo later, when food was scarce. If interspecific competition prevails, finch populations should decline from wet to dry season in response to declining food availability. At 2 sites where food supplies declined sharply, so did an index of finch numbers; at the other 2 sites, both food supplies and finch numbers remained high. Total finch biomass at 5 dry—season study sites was correlated with food abundance but was not at 8 wet—season study sites. If interspecific competition is stronger, diets of finch species should diverge as food becomes scarce, but if intraspecific competition is stronger, diets should expand and overlap among species increase. At all 4 sites, finch species diverged in their diets and took a narrower range of foods in the dry season. All species shifted from a common wet—season diet of soft, easy—to—handle seeds and fruits to different diets reflecting the morphological specializations of each species. The results are consistent with interspecific competition for food occuring at all sides, but other explanations cannot be ruled out. Even where food remained relatively abundant, species diets changed and overlapped less. Intense competition was not, therefore, a necessary condition for seasonal diet changes; these may have been simple proximate responses to food availability. Existing foraging theory does not predict these results or others, because the assumption that available foods do not change qualitatively between seasons is violated. Where food greatly decreased, inter— and intraspecific competition may have caused the death or dispersal of most of the finches in the wet—season populations and diet divergence minimized interspecific competition are probably complemented by year—to—year differences caused by the unpredictable Galapagos rainfall.