Survivorship, Breeding Dispersal and Mate Fidelity in Eastern Kingbirds

Abstract

I measured annual survivorship, breeding dispersal and mate/site fidelity in a color-banded population of Eastern Kingbirds (Tyrannus tyrannus) breeding in central New York from 1989 through 1994. I also sought to identify the factors that led to breeding dispersal and to test whether breeding philopatry was based on site or mate fidelity. Survivorship of males did not differ between the first (0.64) and all subsequent years of banding (0.73), and overall, males returned at a higher rate than females. Female survival in the first year following banding (0.68) exceeded that for all subsequent years (0.51). The difference between the sexes and between banding classes of females may be related to high costs of reproduction in older females. Breeding-site fidelity was very high (96% and 72.5% of males and females, respectively, reused former territories), but females dispersed significantly farther than males between breeding seasons. Site fidelity of males was not reduced significantly by poor nest success in the past year, and was independent of whether or not the female returned to breed. Females often dispersed between seasons after nest failures, but were even more likely to move if the male failed to return. Furthermore, 86.5% of pairs reunited when both partners were known to have returned from migration. Female philopatry thus seemed to be influenced heavily by mate fidelity. Females that renested with a former mate advanced their breeding date by about four days for every year of experience together, but they did not lay larger clutches or fledge more broods than females that nested with a new male. Reunited pairs thus made a rapid transition from migration to egg-laying.