PRODUCTIVITY, FOOD-HABITS, AND BEHAVIOR OF SWAINSONS HAWKS BREEDING IN SOUTHEAST COLORADO

From 1984 through 1988, I studied Swainson's hawk (Buteo swainsoni) ecology during the breeding season on the Pinon Canyon Maneuver Site (PCMS) in southeast Colorado. The number of nesting attempts located and monitored annually ranged from four in 1984, to 22 in 1987. Nests used by Swainson's hawks were located predominantly in one-seed juniper (Juniperus monosperma) or cottonwood (Populus spp.) trees. Traditional nesting success estimates averaged 0.64 and ranged from 0.42 in 1985 to 1.00 in 1984. Mayfield estimates of nesting success ranged from 0.27 (1988) to 1.00 (1984). Based on prey remains collected at nest sites, food deliveries to nestlings consisted primarily of small birds (50%) and mammals (45%), and diet breadth over the 5-yr study period was high. Minimum-convex-polygon home-range size of radio-marked adults during the late-nestling and post-fledging period averaged 21.2 km(2) in 1985 and 27.3 km(2) in 1986, with males exhibiting larger home ranges than females (P = 0.15) across years. Compared with other breeding Swainson's hawk populations, breeding area reoccupancy among years on the PCMS was moderately high, home ranges during the late-nestling and post-fledging periods were large, and ground-nesting birds were important in the breeding-season diet.