ECOLOGY OF BOREAL OWLS IN THE NORTHERN ROCKY-MOUNTAINS, USA

We studied habitat use by boreal owls (Aegolius funereus) in the north ern Rocky Mountains from january through August during 1984-88. Habita t use was examined at several spatial scales. The geographic distribut ion and range of life zones used by boreal owls were documented in wes tern Montana, Idaho, and northwestern Wyoming. Habitat use, at the lev el of the home range, and microhabitats used for nesting, roosting, an d foraging were observed in the Frank Church River of No Return Wilder ness (RNRW) of central Idaho. Boreal owls inhabited forests with in th e spruce-fir (Picea spp.-Abies spp.) life zone throughout the mountain s of Montana, Idaho, and northern Wyoming. Nearly 90% of breeding terr itories located throughout this region were in subalpine fir (Abies la siocarpa) habitat types (based on Steele et al. 1981), and no owls wer e detected below 1,292 m. Within the RNRW, owl breeding sites occurred (n = 28) in mixed-conifer (39%), spruce-fir (25%), Douglas-fir (Pseud otsuga menziesii) (18%), and aspen (Populus tremuloides) (18%) stands. Lodgepole-pine (Pinus contorta) forest, which was the most common veg etation type in the area, was not used for nesting. Nest sites were re stricted to mature and old forest stands with complex physical structu re. Roosting habitat differed between winter and summer. Winter roosts differed little from available forest cover whereas summer roosts had greater canopy cover, higher tree density, and higher basal area than paired random sites. During summer, the owls used cool microsites for roosting; during hot weather, boreal owls frequently exhibited sympto ms of heat stress by gullar fluttering and perching with wings lowered . The best foraging habitat was associated with older spruce-fir stand s. These sites had prey populations 2-10 times greater than other site s and provided open forest structure that facilitated hunting. Because of the wide geographic dispersion of suitable nesting, roosting, and foraging habitat, the owls used large home ranges; ranges averaged 1,4 51 +/- 552 ha in winter (n = 13) and 1,182 +/- 335 ha in summer (n = 1 5). Boreal owls at our intensive study site fed primarily on small mam mals in both winter and summer. During both seasons, southern red-back ed voles (Clethrionomys gapperi) were the most frequent prey, composin g 36% of all prey items. In winter, northern flying squirrels (Glaucom ys sabrinus) were captured by females (14% of prey items) but not male s. During snow-free seasons, boreal owls captured northern pocket goph ers (Thomomys talpoides), yellow-pine chipmunks (Tamias amoenus), and western jumping mice (Zapus princeps) that were unavailable when the g round was snow covered. The size of breeding populations and breeding success varied from year to year. During 1 of 4 years, few owls attemp ted to breed, and we knew of none that raised young. Estimates of demo graphic characteristics of boreal owls suggest that the population dec lined during our investigation. We estimated annual adult survival as 46% (95% CI = 23-91%), and production by nesting females averaged 2.3 (+/- 0.542) young per successful nest. We documented long distance mov ements by adult owls, which support the contention that immigration of nomadic owls may help maintain populations that would otherwise face local extinction. These results suggest that conservation of boreal ow ls will require forest management that maintains the distribution and abundance of mature and older forest stands. Because boreal owls in th e northern Rocky Mountains occur in a narrow life zone, populations ex ist in isolated patches that cover a relatively small portion of the l andscape. To maintain quality habitat at any given site will require s nag retention and timber harvest practices that retain forest structur e. A shift to uneven-age management or modifications of even-age syste ms that retain particular old forest characteristics in spruce-fir for est should meet the owls' needs for nesting structures and roosting si tes while maintaining prey populations.