Fidelity to Breeding Territory in a Population of Red-Tailed Hawks

Abstract

-Fidelity to breeding territories was examined on 33 Red-tailed Hawk (Buteo jamaicensis) territories in Oregon from 1973 to 1982 using the incidence of plumage dimorphism and mortality rates. Evidence suggested a high rate of return to previously occupied territories by females. The rate of change in the color morphs within territories among years was consistent with mortality rates alone. Alternative interpretations involving males' choice of mates with respect to female color and females' selection of habitat with respect to their color were explored. The first possibility, coupled with strong site fidelity by males could not be ruled out although it would require a high degree of color morph preference. No evidence for correlation between female territory selection and color was found. Plumage dimorphism among males occurred too seldom to permit evaluation of breeding site fidelity among males at the population level. Several distinctively marked males were seen, however, to return to previously occupied territories. Hawks are commonly assumed to breed repeatedly on the same territory (Newton 1979). Little evidence for such fidelity exists, however, beyond anecdotes involving a few distinctively marked or otherwise unique individuals. The few recent studies involving marked birds suggest varying degrees of fidelity to breeding territory among species and even sexes (see Newton 1979 for a review). Such behavior has been documented by marked birds in at least three Buteo species (B. galapagoensis, de Vries 1975; B. platypterus, Matray 1974; B. regalis, Schmutz and Schmutz 1981). Observations of distinctively marked individuals have disclosed territory fidelity among Red-tailed Hawks (Buteo jamaicensis; Fitch et al. 1946, Craighead and Craighead 1956). The data for these species do not indicate the degree to which territory fidelity occurs in populations because movements between territories are more likely to pass undetected. I report here on breeding territory fidelity in a population of Red-tailed Hawks, identified with the use of plumage dimorphism. STUDY AREA AND POPULATION Thirty-three territories were monitored by me and from one to six assistants between 1973 and 1982 on a 137-km2 study area in southeastern Wasco County, Oregon. The native vegetation is characteristic of the Columbia Basin Province (Franklin and Dyrness 1973) and forms a mosaic of habitats dominated by bunchgrass (Agropyron spicatum and Festuca idahoensis) or sagebrush (Artemisia tridentata). Dryland wheat farming and grazing, however, have altered much of the native vegetation. In many areas introduced annual grasses (Bromus tectorum and Elymus caput-medusa) and certain shrubs (Chrysothamnus nauseosus, C. vicidiflorus, and Gutierrezia sarothrae) are locally dominant. The Red-tailed Hawks in this study area were dimorphic, 18% (64/358) of the bird-years being represented by dark-phase individuals. One bird residing on a territory during a breeding season represents one bird-year. Darkphase birds were mostly dark below with little or no contrast between the breast and the abdominal band; from beneath, the dark winglining contrasted sharply with the much paler remiges. Light-phase birds were predomiantly white or cream-colored beneath, including the wing-lining and remiges, with a contrasting abdominal band of varying extent. Although lightand dark-phase birds varied considerably, I was able to classify all but one individual into one category or the other. The exceptional bird was an erythristic female, present for three breeding seasons, whom I classified as dark-phase.