A STUDY OF THE WINTER FEEDING HABITS OF THE SHORT-EARED OWL (Asio flammeus) IN THE TORONTO REGION

Abstract

The winter feeding habits of the short-eared owl (Asio flammeus Pontoppidan) were studied during the period of 1936 to 1942 in the vicinity of Toronto, Ont. The owls used restricted roosts in several groups of Douglas fir (Pseudotsuga taxifolia) on a golf course. By means of frequent owl population censuses and collection of pellets, food requirements and correlations were calculated.The winter incursions of the owls were found to be cyclic and to coincide with the local meadow vole (Microtus pennsylvanicus) cycle and to be independent of meteorological factors. Roosting behaviour was found to vary with snow cover.From an analysis of 3000 pellets it was found that the meadow vole formed 82% of the food taken. The next animal most commonly preyed upon was the deer mouse (Peromyscus leucopus), which comprised 17% of the food, while birds formed 1%. No significant seasonal change in diet was noted over a period of five months.It was calculated that a short-eared owl eats between 700 and 1600 mice per year. The average figure is thought to be nearer the minimum estimate. Translated to weight of mice this is equivalent to between 55.5 and 127 lb.It was demonstrated that the amount of snow on the ground affected the relative availability of meadow voles and deer mice. This fact was reflected in changes in pressure on the populations of the two mice species due to owl predation.The data presented pointed to the possibility that concentrations of avian predators could have an appreciable effect on local meadow vole populations. In the case under study it was estimated that winter short-eared owl predation might account for 10% of the vole population.The data also suggested that during periods of deep snow, because of decreased availability of meadow voles, less food is taken and during open periods increased availability of these mice is reflected by increased food consumption.