Behavior and Habitat Use of Breeding Northern Harriers in Southwestern Idaho

Radiotelemetric and visual monitoring of four breeding pairs of the Northern Harrier (Circus cyaneus) in predominantly shrub-steppe habitat of the Snake River Birds of Prey Area, Idaho, indicated that harriers used riparian and cultivated habitats disproportionately. As alfalfa growth approached 46 cm height, males shifted from predating voles (Microtus sp.) in alfalfa fields to the Western Whiptail (Cnemidophorus tigris) in open shrub-steppe habitat. Mean minimum home ranges of males were estimated at 15.7 km² while those of females were estimated at 1.13 km². Males ranged up to distances of 9.5 km from nests. Males hunted most intensively in the second week after hatching. Females did not hunt until the third week after hatching. During most of the day, both sexes rested or preened <0.5 km from nests. Varying home range sizes in this and other studies may be a function of harrier responses to differing habitat structures and prey availability or vulnerability to capture.