PARTIAL MIGRATION AND DIFFERENTIAL WINTER DISTRIBUTION OF HOUSE FINCHES IN THE EASTERN UNITED-STATES

Three hypotheses that attempt to explain latitudinal sex ratio variation on wintering grounds are: 1) the behavioral dominance hypothesis, 2) the arrival time/sexual selection hypothesis, and 3) the body size/physiological hypothesis. Testing these hypotheses is difficult because many predictions are not mutually exclusive. By examining migratory behavior and winter distributions of House Finches (Carpodacus mexicanus), a species in which females dominate males despite their smaller size, we controlled for confounding effects of body size on behavioral dominance. We examined U.S. Fish and Wildlife Service records of House Finches banded throughout the eastern United States following their introduction in 1940. These data suggest the evolution of partial migration among the introduced population of finches, with a trend in recent years (1975-1989) for females to migrate farther than males. The proportion of males on wintering grounds varied significantly and consistently with latitude in the East. Sex ratios north of 38-degrees-N latitude were male-biased, while those south of this were female-biased. Similar patterns were not apparent among western House Finches during winter or in the eastern population during the breeding season. Of the three hypotheses, only the body size hypothesis correctly predicts the pattern observed among wintering finches. Therefore, different energetic constraints facing male and female House Finches, perhaps related to body size, may be influencing migratory behavior and winter distributions.