Some Comparisons of the Feeding Ecology of Four Owls in North-Central Colorado

Abstract

Four species of owls (Great Homed Owl, Bubo virginianus; Long- eared Owl, Asio otus; Burrowing Owl, Speotyto cunicularia and Barn Owl, Tyto alba) which inhabit the same grassland habitats in northcentral Colorado were investigated to determine differences and similarities in feeding ecology. Consider- able overlap in prey species was found in the four owls. However, the average estimated prey weight was significantly different for each owl species. Differences in activity periods, foraging methods and predator-prey size relationships may all function in reducing interspecific competition among these owls. During 1967 observations were made on the feeding ecology of the Great Horned Owl (Bubo virginianus), Long-eared Owl (Asio otus), Burrowing Owl (Speotyto cunicularia) and Barn Owl (Tyto alba) on short-grass prairie or interspersed prairie and farmland in the eastern one-fifth of Larimer County, Colorado. Rock outcroppings, canyons and eroded ditches, all abundant in this area, provide nesting sites for Great Horned and Barn Owls. Small groups of trees along water- courses, wind breaks and orchards are utilized for nesting and roosting by Long-eared and Great Horned Owls. Both occupied and abandoned prairie dog towns attract Burrowing Owls during the nesting season. Blue grama (Bouteloua gracilis) and buffalo grass (Buchloe dacty- loides) are dominant grasses. On rocky soils where rock outcroppings appear, mountain mahogany (Cerocarpus montanus) is abundant. Cottonwoods (Populus spp.) are the most abundant trees and occur mostly along watercourses. Only a small percentage of the land at most of the areas studied was under cultivation. Very little published literature exists on the food habits of these owls in north-central Colorado. Reed (1957) examined Barn Owl pellets from Larimer County and Hamilton (1941) made observations on the food habits of Burrowing Owls in Denver County. Nothing, apparently, has been published on the food habits of the Great Horned and Long- eared Owls for this locality.