Seed and Patch Selection by Galapagos Ground Finches: Relation to Foraging Efficiency and Food Supply

Abstract

I investigated the diets of three species of Darwin's ground finches (Geospiza Gould) on Isla Pinta, Galapagos. Diet choice was examined in relation to both the finches' handling efficiency on seed types and the profitability of patches. Handling efficiency on different seed types was related to the size of the finch species. Correspondingly, the two larger finch species generally ignored small seeds with low rewards, and the two smaller finches avoided feeding on seeds apparently too large and hard to crack. In G. fuliginosa, the smallest species, seed choice was correlated with seed depth within the range of seed sizes actually consumed. In addition, the proportion of its diet made up of given seed types was correlated with the profitability of patches within these seeds occurred. Certain seed types providing low individual returns per unit handling time were highly profitable by virtue of their high density within patches. Conversely, large search times between valuable seeds diminished the relative profitability of respective patch types. Diet breadth of G. fuliginosa was investigated in relation to seasonal changes in food supply. Results on seed choice in relation to relative densities of seed types were ambiguous, but a slight shift toward feeding on smaller seeds in the dry season was detected. However, the shift could be explained by the greater profitability of patches of the small seeds in this season. These finches did not generalize their use of patch types, despite a large seasonal decline in food abundance. This result contradicts a prediction of optimal foraging theory. Serval possible explanations for this are discussed, including an hypothesis that when food was abundant finches selected a diet that was less than maximally efficient.