Birds of Indian Subcontinent

Geographic Variation in Sex Ratios of Wintering American Kestrels Falco sparverius

Publication Type:Journal Article
Year of Publication:1991
Authors:Arnold, TW
Journal:Ornis Scandinavica
Date Published:1991
ISBN Number:00305693
Keywords:Cerchneis sparverius, Falco, Falco sparverius, Falco tinnunculus, Falconidae, Tinnunculus sparverius
Abstract: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. For many species of migratory birds, predictions of these three hypotheses are identical. I examined geographic variation in sex ratios of wintering American Kestrels Falco sparverius using Christmas Bird Count data. Female 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 north 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.
Short Title:Ornis Scandinavica
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith