Neotropical-Nearctic migration, differential migration, migration speed, wood warblers, migration timing, sexual dimorphism
It is well established that male songbirds arrive at the breeding grounds before females in spring, presumably to gain an intrasexual advantage by claiming better quality territories, and therefore mates. However, the mechanism by which males precede females is not fully understood. Three hypotheses have been proposed to explain the earlier arrival of males: (1) males winter farther north than females, therefore migrating a shorter distance, (2) males initiate migration earlier than females, and (3) males migrate more quickly than females. This study tests the “males migrate faster” hypothesis for four species of plumage-sexable wood warblers. Five years of banding data collected at a stopover site north of Chicago, as well as data collected from four banding sites in Mississippi and Louisiana, reveal that the mean difference in arrival date between males and females does not increase as the migration progresses northward from the gulf coast to Chicago. Condition index and fat score data at an Illinois stopover site show no significant difference between the sexes at time of arrival at the stopover site, suggesting that males are not making fewer and/or shorter stopovers during migration. Furthermore, males and females at the same stopover site show no significant difference in the effect of temperature and wind speed on weather-dependent migratory flight decision rules. These three pieces of evidence suggest that males do not migrate more quickly than females. Therefore, males must either winter farther north, migrate earlier, or both.
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