Sex-based differential migration: An examination of proximate causes and ecological consequences

Sarah Elizabeth Mabey

Abstract

Sex-based differential migration in birds has long been considered a product of intra-sexual competition for reproductive resources and inter-sexual competition for survival resources. However, hypotheses explaining differential migration have primarily focused on reproductive constraints of the larger, "territory-holding" sex in the form of same-sex competition for breeding territories and mates and the dominance and physiological advantages that accrue to the larger sex. For songbirds, this has translated into a neglect of the fitness costs and benefits experienced by the females of migratory species. To fully understand the underlying causes of sex-based differential migration, it is essential to consider the selective pressures acting on both males and females and examine the ultimate and proximate factors contributing to the optimal migration strategy of each sex. At stopover sites on the northern Gulf Coast, males passed earlier than females but did not arrive after their Gulf-crossing in better condition than females. These results combine to indicate that, although males may depart from wintering grounds earlier than females or winter in areas closer to the Gulf of Mexico, both males and females were equally able to negotiate crossing a major ecological barrier and equally able to secure resources at these northern Gulf stopover sites. In the Palearctic-African migration system, male muscicapid flycatchers migrated significantly earlier than females, with peak passage for the sexes separated by as much as 10 days. Competition experiments with muscicapid flycatchers revealed that males are socially dominant to females regardless of size and plumage differences. Female Pied Flycatchers (Ficedula hypoleuca ) had greater access to resources when confronted with same-sex competitors than they did with opposite-sex competitors. The endogenous migratory programs of male and female Eastern Kingbirds (Tyrannus tyrannus ) differed significantly in three ways. Although males and females began migrating at the same time, males displayed greater nightly activity than females. The results of this experiment support the hypothesis that males minimize time spent on migration whereas females minimize energy and physiological stress. Descriptive models illustrate the importance of examining the role of migration events in population dynamics and population limitation. (Abstract shortened by UMI.)