Dietary Overlap Between Sympatric Mexican Spotted and Great Horned Owls in Arizona

We estimated diet composition of sympatric Mexican spotted (Strix occidentalis lucida, n = 7 pairs of owls) and great horned owls (Bubo virginianus, n = 4 pairs) in ponderosa pine (Pinus ponderosa)Gambel oak (Quercus gambelii) forest, northern Arizona. Both species preyed on mammals, birds, and insects; great horned owls also ate lizards. Mammals dominated the diet of both species. Mammals comprised 63 and 62% of all prey items identified in diets of spotted and great horned owls, respectively, and 94 and 95% of prey biomass. Both species primarily preyed on a few groups of small mammals. Observed overlap in diet composition between species (0.95) was greater than expected based on null models of diet overlap, and the size range of prey taken overlapped entirely. Mean prey mass was similar for both species (great horned owl, 47.0 ± 7.4 g [SE], n = 94 items; spotted owl, 40.1 ± 1.8 g, n = 1,125 items). Great horned owls consumed larger proportions of diurnally active prey than spotted owls, which primarily consumed nocturnally active mammals. Our results, coupled with a previous analysis showing that these owls foraged in the same general areas (Ganey and others 1997), suggests that they could compete for food resources, which are assumed to be limiting in at least some years. They may minimize the potential for resource competition, however, by concentrating foraging activities in different habitats (Ganey and others 1997) and by foraging at different times, when different suites of prey species are active.