Sex-Biased Winter Distribution and Timing of Migration of Hermit Thrushes (Catharus Guttatus) in Eastern North America

Abstract

Abstract We analyzed distribution of Hermit Thrushes (Catharus guttatus faxoni) wintering in the eastern United States and northeastern Mexico using data from 2,077 specimens collected in winter or during migration. Hermit Thrushes collected in winter showed a sex-biased latitudinal distribution, with the female mean 1.2° south of the male mean. Three general, nonmutually exclusive hypotheses have been proposed to account for that pattern in other species: (1) dominance of males over females, which forces females to lower latitudes; (2) larger body size of males, which permits males to overwinter in colder areas; and (3) earlier arrival by males on breeding grounds, which selects for males to winter closer to the breeding grounds. The dominance hypothesis requires that females be subordinate, an assumption that is not supported by previous research with wintering Hermit Thrushes. We also found no difference in sex ratio among habitats, despite strong differences in habitat quality. The body-size hypothesis requires that the larger sex be able to withstand colder temperatures. Male Hermit Thrushes are larger, but neither multiyear climate data nor weather in the specific month and year when birds were collected showed males to face significantly colder temperatures on the wintering grounds. Our data are most consistent with the arrival-time hypothesis. Males collected during spring migration migrated ∼20 days before females. When males began migrating in March, they experienced colder weather than sedentary females. By wintering farther north and migrating earlier, males expedite their return to their breeding grounds, although the small difference in winter distribution between the sexes is marginal compared to the effect of males’ much earlier departure. Presumably, any cost to males of wintering or migrating under more demanding conditions is balanced by the advantage of early arrival on the breeding grounds.