Demography of Ferruginous Hawks Breeding in Western Canada

Abstract: We assessed age-dependent survival, site-fidelity, and, together with data on prey and reproduction, trends in the population of ferruginous hawks (Buteo regalis) breeding in western Canada. Our analysis included 7,129 ferruginous hawks banded near Hanna, Alberta, and Kindersley-Alsask, Saskatchewan, from 1972 to 2003. We estimated annual adult survival rate to be 0.708 (SE = 0.024) and first year survival for nestlings was 0.545 (SE = 0.147). Resighting probability was modeled as a constant for nestlings (0.009, SE = 0.010), but it varied among years for adults consistent with our sampling efforts. Band reporting rate was at 0.022 (SE = 0.007) for both nestlings and adults. Fidelity to the study site was 1.00 (SE = 0.000) for adults and 0.035 (SE = 0.014) for nestlings. Nesting density ranged from 3.1 to 14.0 pairs/100 km² and averaged 9.8 pairs/100 km². We observed an average clutch size of 3.2 (SE=0.06) and brood sizes of 2.71 (SE=0.07) near Hanna and 2.79 (SE = 0.99) at Kindersley-Alsask. Richardson's ground squirrels (Spermophilus richardsonii) made up 95% of ferruginous hawk prey. Prey availability was positively correlated with number of offspring near Hanna and Kindersley-Alsask. We believe the lower than expected adult survival did not result in population decrease; rather, declines in reproduction resulting from declines in the abundance of ground squirrels better explain an observed 4.5-fold decline in nesting densities during the study. The results suggest that ferruginous hawk management should address prey in addition to habitat management, and that management needs are regional in scope with particular emphasis on the breeding range within the northern Great Plains.