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Food niches of Common Barn-Owl (Tyto alba) populations were studied for 8 years in Utah and Idaho. Small mammals dominated the diet as they do in most parts of the barn-owl's range. Mean prey weight was nearly identical between the two owl populations. Food-niche breadth, though, was significantly broader in Idaho than in Utah. Food-niche breadth was also more variable in Idaho than in Utah both within and between years. Voles (Microtus spp.) were predominant prey for both owl populations and were the only prey whose numeric variation in the diet significantly affected mean prey weight and food-niche breadth. Vegetative composition was less varied and more uniformly favorable to voles in Utah. The Idaho area contained a smaller poportion of good vole habitat but had a greater diversity of vegetation including deserts suitable for small mammal species not found in the Utah study area. These vegetative differences and their effects on small mammal species diversity and density were the most apparent causes of food-niche differences between the two owl populations.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Cooper Ornithological Society is collaborating with JSTOR to digitize, preserve and extend access to The Condor. This paper describes variety and size of prey of the Long-eared Owl (Asio otus) from widespread parts of its range and develops a diet profile for the species. Compared to sympatric species, the Long-eared Owl has a restricted diet (Craighead and Craighead 1956, Errington 1932, Korschgen and Stuart 1972, Marti 1974, Maser et al. 1970). Food habits of this species have been extensively reported from North America and Europe. There are, however, large areas within its range from which no reports on diet are available. The Long-eared Owl is Holarctic in north temperate regions of North America, Europe and Asia (see Grossman and Hamlet 1964). This is a strictly nocturnal species (Marti 1974) and apparently feeds almost exclusively in open lands (Getz 1961, Randle and Austing 1952, Smeenk 1972, Weller et al. 1963). However, it does require small, dense trees for nesting and roosting (Armstrong 1958, Bent 1938, Marti 1974). This species is a medium-sized owl; mean weight of 66 specimens from North America was 262.3 g (Earhart and Johnson 1970). Several morphological adaptations point to efficiency of hunting in open areas. The long wings seem to be adapted to this habitat (Lack 1966), and compared to many other North American owls this species has rather light wing-loading, which indicates efficiency of hunting on the wing. Poole (1938) calculated wing-loading for the Long-eared Owl as 5.13 cm2 of wing area per gram of body weight for males and 4.22 for females. METHODS AND MATERIALS The analysis of prey variety and size is based on data from studies listed in table 1. Most weights WOOLLER, R. D., AND G. S. TRIGGs. 1968. Food of the Long-eared Owl in Inverness-shire. Bird Study 15:164-165.