Greater Land Bird Densities on Island vs. Mainland: Relation to Nest Predation Level

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Abstract
Densities of Black—throated Sparrows (Amphispiza bilineata) are greater on Coronados Island in the Gulf of California than on the adjacent Baja peninsula, while those of Verdins (Auriparus flaviceps) and Costa's Hummingbirds (Calypte costae) are not. I tested the hypothesis that the higher sparrow densities on the island were influenced by greater nesting success there. Nesting success for 1984 and 1985 combined was significantly higher on the island than the adjacent Baja California peninsula for the sparrows, but not for Verdins or hummingbirds. Predation on artificial nests placed in sites similar to those used by Black—throated Sparrows was significantly lower on the island. Damage to eggs in artificial nests indicated that birds were responsible were responsible for at least some of the nest failures; correspondingly, densities of avian nest predators were significantly higher on the mainland. These data are consistent with the hypothesis that higher densities of Black—throated Sparrows are due to reduced predation on the island, and suggest that processes affecting density differences between island and mainland locations may differ among species.