Differential timing of spring migration in wood warblers (Parulinae).

Spring migration patterns of 18 species of paruline warbler at Prince Edward Point, Ontario showed that males arrived earlier than females in all species. Adult males arrived significantly earlier than second-year males in American Redstarts (Setophaga ruticilla), and there was evidence for a similar trend in other species. The difference in mean arrival dates between the sexes was greatest in species that arrived earliest. Similarly, within species, the difference between sexes was greatest in years when the males arrived earliest. For individuals within a species there was a significant negative correlation between arrival date and wing length; however, males of a particular size generally arrived earlier than females of the same size. Thus, larger size may be an advantage to early arrival, but is not sufficient to explain the difference in arrival between sexes. Species that winter furthest north arrived earliest, but sexual differences in wintering grounds have not been reported. These results are consistent with the hypothesis that males are selected to arrive as early as food resources or climatic conditions are adequate, whereas females arrive later, closer to the time when they can successfully begin nesting.