A Mechanism of Sexual Segregation by Habitat in American Kestrels (Falco sparverius) Wintering in South-Central Florida

I observed the establishment of 240 winter territories by American Kestrels (Falco sparverius) on a 293-km census route in south-central Florida during autumn 1985. Most females arrived before males. Analysis of banding records revealed that immatures of both sexes and adult females preceded adult males in autumn migration in eastern North America; thus, the sex ratio of early arrivals was skewed toward females. In south Florida territorial kestrels occupied habitats in decreasing order of foraging quality, as measured by percentage cover of suitable hunting substrate (grasses or weedy forbs < 25 cm in height) and woody canopy cover (which was correlated negatively with suitable hunting substrate and obstructed the view of hunting kestrels). Early-arriving males also occupied habitats of superior foraging quality and were as successful as females in defending territories against same-sex and opposite-sex kestrels as high-quality habitats became limited. Results of experiments in which free-flying intruder kestrels were released onto defended territories suggested that males defended winter territories more tenaciously than did females. Because there was no evidence of male submissiveness on the wintering grounds, the female-dominance hypothesis is not a plausible explanation for sexual segregation by habitat in wintering kestrels. Each kestrel's arrival date was apparently the principal determinant of the foraging quality of habitats still available for occupancy; foraging quality was correlated negatively with arrival date for the adult males and for the females and immatures. A delayed molt in adult males, associated with differential sex roles on the breeding grounds, may delay the migratory departure, resulting in the late arrival on the wintering grounds.