Annual Variability in the Abundance of Migrant Landbirds on Southeast Farallon Island, California

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Abstract
I analyzed the occurrences of 217 species of migrant landbirds on Southeast Farallon Island, 43 km west of San Francisco, California, from 10 yr of daily censuses, fall 1968 to spring 1978. I divided species into 12 subgroups to reflect seasonal and geographical distributions on the mainland. The abundances of the various subgroups on the Farallones, in both fall and spring, were directly related to the proximity to the island of their breeding and wintering ranges and normal migration routes. For most subgroups, abundances were greater in fall than in spring. The great majority of fall individuals were hatching-year birds, and I suspect that many, if not most, spring individuals were second-year birds. Highly significant year-to-year variations occurred in the number of individuals that arrived on the Farallones and in the proportion of individuals in the various subgroups, despite the fact that the number of species and proportion of species in the various subgroups remained statistically invariant from year to year. Of particular interest were 5 fall days and 4 spring days when exceptional numbers of birds arrived on the island. These wave days were probably the result of local weather conditions near the Farallones. The numbers of individuals of most subgroups of wintering and summering species, especially coastal subgroups, increased significantly on wave days, but the numbers of vagrants did not. The annual variations in abundance of Farallon landbirds were probably caused by variations in the number of young produced each summer, by long-range weather patterns acting over large areas of the continent, and by variations in local weather conditions. Vagrant species significantly increased in abundance in both fall and spring during the second 5 yr of this study. I suggest that this could be caused by a selective increase in the proportion of dispersing individuals (including vagrants) in populations subjected to increased rates of habitat change.

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