Habitat utilization by spotted owls in the West-Central Cascades of Oregon Public Deposited



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  • Eight adult spotted owls (Strix occidentalis) on the west slope of the Cascade Range in Oregon were fitted with radio transmitters and observed for periods ranging from 271 to 383 days. Mean home range size for the owls was 1,177 ha (range = 920-1,376 ha). There was no significant difference between home range size of males and females. On the average, home ranges of individuals occupying adjacent territories overlapped by 12 percent (range = 3-25 percent). Home ranges of paired individuals overlapped by 50 to 73 percent. The size and shape of home ranges varied on a seasonal basis; generally, the largest home ranges were observed during winter. Nest sites were centrally located within the home ranges utilized by the owls. On the radiotelemetry study area, the mean nearest neighbor distance between nests of adjacent pairs was 2.88 km. On 11 other areas in western Oregon where suitable habitat was available, mean nearest neighbor distances ranged from 1.68 to 3.04 km. The overall mean nearest neighbor distance for the 12 areas was 2.42 km. Home ranges occupied by the radio tagged owls were so large that consistent defense of the entire home range was impossible. Territorial defense appeared to take the form of a decreasing zone of influence centered around the nest; areas near each nest were defended consistently, but defense became increasingly inconsistent with distance from the nest. Discrete territorial boundaries could not be determined and probably did not exist. Territorial interactions between individuals of the same sex were much more aggressive than interactions between individuals of opposite sex. Spotted owls may have responded less aggressively toward individuals of the opposite sex because the advantage of allowing potential mate replacements to remain within the territory outweighed other considerations. Except for a few instances in which 2 of the owls foraged in recently clear-cut areas or in rock talus, virtually all foraging occurred in forests over 30 years old. Old-growth forests, which covered 36 to 64 percent of the area on individual owl home ranges, were strongly preferred for foraging by all of the owls; 92 percent of all foraging occurred in such forests. Utilization of younger forests (30 to 200 years old) was generally either less than or not significantly different than expected, indicating that younger forests were less desirable for foraging than old-growth stands. Old-growth forests were also preferred for roosting (97 percent of all roosts). Large old trees apparently were preferred for roosting during inclement weather because they provided better overhead protection from rain and snow. Small trees in the understory were preferred during warm weather because they provided greater protection from high temperatures and solar radiation. Spotted owls were primarily nocturnal. On the average, they left their day roasts to begin foraging at 14 minutes after sunset and stopped foraging at 21 minutes before sunrise. Diurnal foraging was limited primarily to opportunistic attempts to capture animals that wandered into roost areas. The usual method of foraging was to move from perch to perch at night, watching and listening for potential prey. The average rate of movement while foraging in this manner was 247 m/hr, and the average distance moved per nightly foraging period was 2,782 meters (range = 320-8,895 m). Seven species of mammals comprised the nucleus of the diet, the northern flying squirrel (Glaucomys sabrinus), red tree vole (Phenacomys longicaudus), western red-backed vole (Clethrionomys occidentalis), deer mouse (Peromyscus maniculatus), western pocket gopher (Thomomys mazama), snowshoe hare (Lepus americanus), and bushy-tailed woodrat (Neotoma cinerea). The flying squirrel, the most common animal in the diet, comprised 42 percent of all prey captured. During mid-winter, 85 percent of the prey captured were arboreal mammals. During the rest of the year, terrestrial prey became more abundant in the diet, but the flying squirrel remained the most common animal in the diet. Mean prey size and dietary composition for male and female spotted owls were not significantly different; this suggested that males and females did not partition the prey resource on the basis of size or species. Estimates of biomass consumed per owl per day on the study area ranged from 77.6 to 164.8 g, but the latter estimate was probably inflated. Four hypotheses are presented to explain why spotted owls foraged primarily in older forests and avoided recently cutover areas: (1) the biomass of prey preferred by the owls may have been greater in older forests than in cutover areas, (2) switching from arboreal mammals in older forests to terrestrial mammals in cutover areas might have resulted in increased competition with other nocturnal predators, (3) prey may have been less accessible in the dense vegetation that developed on cutover areas, and (4) spotted owls may have been more susceptible to predation in cutover areas where escape cover was absent.
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