Feeding Correlates of Breeding and Social Organization in Two Galápagos Finches

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Abstract
Several aspects of breeding and social behavior in two Galápagos finches are related to food supply. Geospiza fuliginosa is largely granivorous, and G. difficilis is largely insectivorous. Measurements show that breeding of both species occurred when arthropod abundance was high. Breeding stopped when arthropod abundance was declining but before it was low. Breeding-territory size in G. difficilis varied inversely with food supply. Territory size in G. fuliginosa was similar across sites and was not correlated with food supply. This difference between species may be attributed to differences in the effects of invertebrate and seed resources on the functional response of consumers: feeding rate of G. difficilis on invertebrates increased linearly with their supply, whereas feeding rate of G. fuliginosa on seeds was essentially independent of seed abundance. In the nonbreeding season, the granivore G. fuliginosa moved in flocks, whereas G. difficilis was more stationary and solitary. Data indicate that seed supply was temporally more variable than invertebrate abundance, and this helps account for dry-season behavioral differences between the species. Functional response and/or renewability of seeds and invertebrates may also have influenced behavior at that time.

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