Shook, John E. – Breeding biology, nesting habitat, dietary analysis and breeding behaviors of Northern Hawk Owls (Surnia ulula) in interior Alaska
Most data recorded on northern hawk owls (Surnia ulula) is from Fennoscandia and very few studies have investigated this species in North America during the breeding season. I collected breeding season data from 21 breeding pairs, 11 nonbreeding pairs, 31 unpaired adults and 67 young with the objective to quantify hawk owl breeding biology, nesting habitat characteristics and diet, and to describe their breeding behaviors. I located 21 nests, and hawk owls showed no nest site fidelity over the 3 years of this study. Nests were in three major forest types (needleleaf, broadleaf, and mixed) and all nests had open tree canopy cover (x = 23.7%), while 95% of nests had open shrub canopy cover (x = 35.0%).
Because 96% of the diet of hawk owls consisted of microtine rodents and snowshoe hares (Lepus americanus) by both frequency and biomass, these major prey categories were used for comparisons. Percent frequency and percent biomass of microtines, average prey biomass and diet evenness differed among forest types (P < 0.04). Results of observational data was not different than results of pellet analysis (P > 0.6), suggesting that the use of pellet analysis is an accurate measure of hawk owl diet. The three predators that seemed to elicit the strongest responses were the great horned owl (Bubo virginianus), northern goshawk (Accipiter gentilis), and lynx (Lynx canadensis) and previous studies have shown that these predators prey on hares and their populations track hare abundances. Hawk owls also preyed on hares in this study at varying amounts (0-93% biomass) and this implies that increased risk of predation may be a cost of nesting in areas with high numbers of hares. Hawk owls ate more hares in areas with high hare densities, and microtine species in the diet of owls appeared to correspond with each species spatial distribution and habitat associations. This suggests that hawk owls in interior Alaska are not selective hunters, but rather hunt opportunistically, and this is contrary to the literature from Fennoscandia.