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TROPHIC NICHE OF NEARCTIC SHORT-EARED OWLS

AssTticr.-The trophic niche of Short-eared Owls (Asiojlammeus) was analyzed using nine Nearctic studies reporting > 500 prey items each. Of 20,416 prey items, 4136 were from the breeding and 16,280 from the non-breeding seasons. The owls preyed upon at least 62 species from four classes of animals. Mammals constituted > 95% of prey from all but two sites. Food-niche breadth ranged from 1.23 to 5.20 (Z = 1.87), for combined studies, 1.3 1 to 1.87 (K = 1.50) for breeding, and 1.23 to 5.20 (3 = 2.00) for non-breeding seasons. Dietary evenness values ranged from 0.315 to 0.703 (.Z = 0.435), for combined studies, 0.331 to 0.404 (Z = 0.365) for the breeding season, and 0.315 to 0.703 (X = 0.458) for the non-breeding season. Estimated prey masses ranged between 28 and 325 g. Most prey, however, weighed between 28 and 100 g. Short-eared Owls from coastal areas preyed more on birds than those at inland sites. Received 9 May 1992, accepted 24 Dec. 1992. Despite numerous studies of the diet of the Short-eared Owl (Asio flammeus), there has been no quantitative review of their trophic niche in the Nearctic zoogeographical region. Clark and Ward (1974) calculated prey diversity for the Short-eared Owl, using the published literature but did not state the criteria they used for assigning prey categories or go into much discussion of its meaning. Here, I summarize the trophic niche of Short-eared Owls from nine Nearctic studies. My objectives were to (1) compare trophic niche among studies, (2) compare trophic niche between breeding and non-breeding seasons, and (3) estimate prey size. METHODS I defined the trophic niche as the relationship between the owls and their food. I followed Marti’s (1987) definitions for trophic diversity where a broad food-niche breadth has high prey species numbers which are nearly equally distributed and a narrow food-niche breadth has few prey species numbers unequally distributed. I estimated trophic diversity by calculating food-niche breadth and dietary evenness. I compared breeding season (N = 3) and non-breeding season (N = 9) diets from studies with >500 prey items. I combined crustaceans, insects, and birds to the class level because species or genera were not always identified. To compare trophic niche among studies, I used only mammals identified to species. I did so because they represented >95.0% of prey from all but two studies. Foodniche breadth (FNB) was calculated using the antilog of the Shannon-Wiener diversity index because it is related linearly to the number of prey categories in the sample and is easy to interpret (Marti 1987). Evenness was calculated using Alatalo’s (198 1) modification of Hill’s (1973) equation: Evenness = (N, - l)/(N, - l), where N, = exp H’ and N, = l/Z p,*.