Differential Habitat use by Sexes of American Kestrels Wintering in Northern California
ABSTRACT
Differential habitat use by sexes of American Kestrels (Falco sparverius) wintering in Humboldt, Mendocino, Lake, Colusa and Yolo Counties, California, was discovered and quantified by roadside counts conducted during the winters of 1970-71 through 1972-73. A prevalence of females was discovered in expansive pastures and fields devoid of or with few trees, and a prevalence of males was discovered in orchards and small pastures and fields surrounded by trees. However, Chi-square tests of homogeneity and frequency indicated a complex transition in habitat use. Relative abundance of Kestrels, measured by linear distance between 340 successive observations, differed significantly only between the Central Valley and the other areas sampled. Kestrels averaged 115 individuals per 100 miles (161 kin; 135 observations) in the Central Valley and 34 birds per 100 miles (161 km; 205 observations) elsewhere. The differential use of habitats by wintering Kestrels is interpreted as character displacement. Its relevance to the principle of competitive exclusion is discussed.
