Owl winter irruptions as an indicator of small mammal population cycles in the boreal forest of eastern North America

Contrary to what is observed in Fennoscandia, it seems to be widely accepted that small mammals do not exhibit multi-annual population cycles in the boreal forest of North America. However, in the last thirty years, irruptions of vole predators such as owls have been reported by ornithologists south of the North American boreal forest. While such southerly irruptions have been associated in Fennoscandia with periods of low abundance of small mammals within their usual distribution range, their possible cyclic nature and their relationships to fluctuations in vole densities at northern latitudes has not yet been demonstrated in North America. With information collected from existing data-bases, we examined the presence of cycles in small mammals and their main avian predators by using temporal autocorrelation analyses. Winter invasions of boreal owls (Aegolius funereus ) were periodic, with a 4-yr cycle in Québec. Populations of one species of small mammal, the red-backed vole (Clethrionomys gapperi ), fluctuated periodically in boreal forests of Québec (north to 488N). Boreal owls show invasion cycles which correspond to years of low density of red-backed voles, the main food item for this owl species. In addition, winter observations of northern hawk owls (Surnia ulula ) and great gray owls (Strix nebulosa ) south of their usual range increased in years of low density of red-backed voles. Our results suggest that a 4-yr population cycle exists in the eastern boreal forest of North America for voles and owls, which is very similar to the one observed in Fennoscandia.