Habitat utilization by spotted owls in the west-central Cascades of Oregon

Eight adult spotted owls (Strix occidentalis) on the west slope ofthe Cascade Range in Oregon were fitted with radio transmitters andobserved for periods ranging from 271 to 383 days. Mean home range sizefor the owls was 1,177 ha (range = 920-1,376 ha). There was nosignificant difference between home range size of males and females.
On the average, home ranges of individuals occupying adjacent territoriesoverlapped by 12 percent (range = 3-25 percent). Home ranges of pairedindividuals overlapped by 50 to 73 percent. The size and shape of homeranges varied on a seasonal basis; generally, the largest home rangeswere observed during winter. Nest sites were centrally located withinthe home ranges utilized by the owls.
On the radiotelemetry study area, the mean nearest neighbordistance between nests of adjacent pairs was 2.88 km. On 11 other areasin western Oregon where suitable habitat was available, mean nearestneighbor distances ranged from 1.68 to 3.04 km. The overall meannearest neighbor distance for the 12 areas was 2.42 km.
Home ranges occupied by the radio tagged owls were so large thatconsistent defense of the entire home range was impossible. Territorialdefense appeared to take the form of a decreasing zone of influencecentered 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 didnot exist.
Territorial interactions between individuals of the same sex weremuch more aggressive than interactions between individuals of oppositesex. Spotted owls may have responded less aggressively towardindividuals of the opposite sex because the advantage of allowingpotential mate replacements to remain within the territory outweighedother considerations.
Except for a few instances in which 2 of the owls foraged inrecently clear-cut areas or in rock talus, virtually all foragingoccurred in forests over 30 years old. Old-growth forests, whichcovered 36 to 64 percent of the area on individual owl home ranges,were strongly preferred for foraging by all of the owls; 92 percent ofall foraging occurred in such forests. Utilization of younger forests(30 to 200 years old) was generally either less than or not significantlydifferent than expected, indicating that younger forests were lessdesirable for foraging than old-growth stands.
Old-growth forests were also preferred for roosting (97 percent ofall roosts). Large old trees apparently were preferred for roostingduring inclement weather because they provided better overheadprotection from rain and snow. Small trees in the understory were preferred during warm weather because they provided greater protectionfrom high temperatures and solar radiation.
Spotted owls were primarily nocturnal. On the average, they lefttheir day roosts to begin foraging at 14 minutes after sunset andstopped foraging at 21 minutes before sunrise. Diurnal foraging waslimited primarily to opportunistic attempts to capture animals thatwandered into roost areas. The usual method of foraging was to movefrom 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 2,782meters (range = 320-8,895 m).
Seven species of mammals comprised the nucleus of the diet, thenorthern flying squirrel (Glaucomys sabrinus), red tree vole(Phenacomys longicaudus), western red-backed vole (Clethrionomys occidentalis), deer mouse (Peromyscus maniculatus), western pocketgopher (Thomomys mazama), snowshoe hare (Lepus americanus), and bushy-tailed woodrat (Neotoma cinerea). The flying squirrel, the most commonanimal in the diet, comprised 42 percent of all prey captured. Duringmid-winter, 85 percent of the prey captured were arboreal mammals.During the rest of the year, terrestrial prey became more abundant inthe diet, but the flying squirrel remained the most common animal inthe diet.
Mean prey size and dietary composition for male and female spottedowls were not significantly different; this suggested that males andfemales did not partition the prey resource on the basis of size orspecies. 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 probablyinflated.
Four hypotheses are presented to explain why spotted owls foragedprimarily in older forests and avoided recently cutover areas: (1) thebiomass of prey preferred by the owls may have been greater in olderforests than in cutover areas, (2) switching from arboreal mammals inolder forests to terrestrial mammals in cutover areas might have resultedin increased competition with other nocturnal predators, (3) prey mayhave been less accessible in the dense vegetation that developed oncutover areas, and (4) spotted owls may have been more susceptible topredation in cutover areas where escape cover was absent.