Winter Distribution of Age and Sex Classes in an Irruptive Migrant, the Evening Grosbeak (Coccothraustes vespertinus)

Abstract

I used bird--banding data (1955-1988) and logistic regression analyses to test the prediction that male Evening Grosbeaks (Coccothraustes v. vespertinus) winter farther north than females, and to investigate whether age groups (first-winter vs. older) differ in their winter distribution. Males wintered farther north than females in 28 of 31 winters in eastern North America (<85°W), and in 12 of 31 winters in central regions. Males typically comprised 53-65% of individuals wintering at the northern edge of the winter range, and 18-27% in the south. No differences in distribution between age classes were observed. Differential migration of the sexes in this species is most likely related to social dominance behavior or to the ability of the larger-bodied males to tolerate harsh winter conditions in northern regions.