Geographic variation in sex ratios of wintering American kestrels Falco sparverius

Three general hypotheses have been advanced to account for latitudinal variation in wintering ranges between the sexes in migratory birds: 1) the larger-bodied sex can endure harsher climates and therefore winters farther north, 2) behavioral dominance forces the subordinate sex to migrate farther south to avoid competition, and 3) intrasexual selection favors early arrival on breeding territories by individuals of one sex, and members of that sex winter farther north to maintain closer proximity to the breeding grounds. Geographic variation in sex ratios of wintering American kestrels was examined using Christmas Bird Count data. Females kestrels are larger and presumably more dominant than males, but they do not arrive first on the breeding grounds. Contrary to the body-size and behavioral-dominance hypotheses, male kestrels wintered slightly N of females. However, a review of data from other species with reversed sexual size dimorphism indicates that the arrival-time hypothesis cannot account for all cases of latitudinal segregation between the sexes. -from Author