Nesting Success and Prey Selection of Long-Eared Owls along a Juniper/Sagebrush Ecotone in Southcentral Idaho

Abstract

A population of Long-eared Owls (Asio otus) nesting in a juniper/sage- brush ecotone had an average density of one pair per 0.65 km2 and fledged an average of 3.7 young per nest. All 24 nests were built by corvids and were located in juniper trees. Mammals comprised 95.2% of 1000 prey remains collected. The proportion of Heteromyidae and Cricetidae taken as prey by Long-eared Owls was significantly different (x2 = 27.2, 3 df, p < 0.005) from the small mammal composition of the area. Deer mice (Peromyscus maniculatus) and pocket mice (Perognathus parvus) occurred in the same relative frequencies in pellets and in trapping studies, while kangaroo rats occurred twice as often in pellets as in traps and harvest mice occurred less than half as frequently in pellets as in traps. The objective of this research was to determine nesting density, productivity, and prey utilization of Long-eared Owls (Asio otus) nesting along a juniper (Juniperus osteo- sperma)/sagebrush (Artemisia tridentata) ecotone in southcentral Idaho. This habitat is common throughout much of the West and is an important breeding area for many raptors because of the proximity of suitable nesting sites to hunting areas. Long-eared Owls are well-suited to this habitat since they hunt in open areas (Getz 1961) and nest and roost in dense clumps of trees (Armstrong 1958, Bent 1938). Few detailed studies of Long-eared Owls have been conducted on the rangelands of western North America (Marti 1974). Studies that have been done in this region were primarily located in riparian habitat with deciduous trees or near agricultural development (Sonnenberg and Powers 1976, Craig and Trost 1979, Marks and Yensen 1980). Prior research on Long- eared Owls has not focused on use of the juniper/sagebrush ecotone, a habitat which this study shows is used extensively by this species. In fact, Long-eared Owls were the most abundant raptor in the study area where this type of habitat was present (Thurow et al. 1980).