NORTHERN HAWK-OWLS IN THE NEARCTIC BOREAL FOREST - PREY SELECTION ANDPOPULATION CONSEQUENCES OF MULTIPLE PREY CYCLES

We studied hawk-owls in the southwestern Yukon, Canada, from 1987-1993 . Most information on hawk-owls originates from studies in Europe, and very little is known about the subspecies Surnia ulula caparoch in No rth America. The boreal forest communities in the two continents diffe r remarkably in the composition of cyclic herbivore populations. Fenno scandia is dominated by 3-4 year microtine cycles, whereas northern Ca nada and Alaska experience a 10-year cycle in snowshoe hare numbers, w ith voles fluctuating at lower levels, We studied the diets of nine ne sting pairs by pellet analysis, and we observed prey deliveries at fiv e nests. The proportion of voles in the diets was lower than reported from Fennoscandia, and snowshoe hares made up 40-50% during the peak o f the hare cycle, Estimates of prey densities by live-trapping reveale d that hawk-owls strongly prefer voles over snowshoe hares and squirre ls. Among voles, Microtus were preferred and Clethrionomys were avoide d. Hawk-owls showed, however, a functional response not only to voles but also to juvenile hares, and they may be critically dependent on la rger prey during certain nesting stages when vole abundance is moderat e or low. Breeding densities and winter observations changed concurren tly over years of different prey abundance, Prey selection Translated into population consequences: hawk-owls did not respond numerically to Clethrionomys outbreaks, but to the combined densities of Microtus an d snowshoe hares. We conclude that the Northern Hawk-Owl is less of a vole specialist and more affected by the prey composition in specific systems than commonly assumed, and we discuss this pattern from an evo lutionary perspective.