Rough-legged hawk winter ecology in Southeastern Idaho
Watson, James Ward
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Ecology of the American rough-legged hawk (Buteo lagopus) was investigated during winters 1981-82 and 1982-83. Research was conducted in sagebrush-dominated habitat on the Idaho National Engineering Laboratory Site in southeastern Idaho. Spatial distribution of the hawk population was influenced by vegetation canopy, which reduced foraging efficiencies of monitored hawks. Temporal distribution was influenced by rabbit carrion availability and snow cover over 10 em, which accounted for 65% of the variability in hawk numbers. Hawks consumed carrion and voles in different proportions on sagebrush habitat and adjacent farmland. Radio-tagged hawks exhibited two patterns of range use. Three hawks maintained a pattern of drift between small, non-overlapping ranges. Four hawks occupied well-defined ranges between 70.2 and 541.2 km ² which encompassed smaller, overlapping weekly ranges. Weekly ranges were characterized by size reduction in mid-winter due to increased perching activities which conserved energy. Monitored hawks also shifted to grass-dominated areas or highways in the period of severe weather. Transmission lines were important components of ranges. Utility poles influenced range shape and size and were used extensively as hunting and roosting. substrates. Hawks exhibited little range defense but defended perches and prey near activity centers. Other aspects of social behavior, hunting success, roosting behavior and food habits were reported. Premigratory movements up to 120 km from ranges were exhibited by three hawks. Movements of two hawks averaged 120 km per day for two days of migration. Winter range fidelity was displayed by at least one individual. The presence of marked hawks resighted in states surrounding Idaho reflected the opportunistic nature of this species. Theoretical considerations suggested this population originates in western Canada.