Northern Spotted Owls: Influence of Prey Base and Landscape Character
Authors: Carey Andrew B. , Horton Scott P. , Biswell Brian L. 
Published: April 2006

We studied prey populations and the use and composition of home ranges of 47 Northern Spotted Owls (Strix occidentalis caurina) over 12 mo in five landscapes in two forest types in southwestern Oregon. We measured 1—yr home ranges of 23 owl pairs, 2—yr home ranges of 13 pairs, and 3—yr home ranges of 3 pairs. The landscapes differed in the degree to which old forest had been fragmented by wildfire and logging. Prey populations were measured at 47 sites in southwestern Oregon. Further data on prey populations were gathered on 14 sites on the Olympic Peninsula in northern Washington, where owls use larger ranges than in Oregon. Owls in Washington used ≈1700 ha of old forest annually and primarily one prey species; available prey biomass was 61 g/ha. Owls in Oregon Douglas—fir (Pseudotsuga menziesii) forests used 813 ± 133 ha (X ± SE) of old forest annually and concentrated on two prey species that had a combined biomass of 244 g/ha. Owls in Oregon mixed—conifer forest used 454 ± 84 ha of old forest annually and three primary prey whose availability averaged 338 g/ha. The amount of old forest used by owls studied for 2 yr was 40% greater in the 2nd yr than that used in the 1st yr. No increase in use of old forest was seen in the 3rd yr in Douglas—fir forest; 50% more old forest was used in 3 yr than in the 1st yr in mixed—conifer forest. The most common prey in Washington and Oregon was the northern flying squirrel (Glaucomys sabrinus). In areas where the flying squirrel was the primary prey and where predation was intense (as judged by telemetry), flying squirrel populations were depressed. The addition of medium—sized mammal species, especially woodrats (Neotoma spp.), to the prey base appeared to reduce markedly the amount of old forest used for foraging. Owls traversed 85% more Douglas—fir forest and 3 times more mixed—conifer forest in the heavily fragmented areas than in the lightly fragmented areas. Overlap among pairs and separation of birds within pairs in space increased with fragmentation. In the most heavily fragmented landscape, social structure appeared to be abnormal, as judged by the proportion of adult—subadult pairs, instances of adult nomadism, and overlap among the home ranges of pairs. The pattern of fragmentation affected the ability of owls to find concentrations of old forest in the landscapes. Even so, almost all the owls consistently selected old forests for foraging and roosting; only one owl selected a younger type as part of its foraging range. Selection of old forest was significant at three levels: landscape, annual home ranges of pairs, and foraging and roosting sites of individuals. The most important prey species, the northern flying squirrel, was twice as abundant in old forest as in young forest in all areas. Landscape indices (dominance, contagion, variance in density of old forest) had less predictive ability than indices based on owl home ranges because owls selected areas of concentrated old forest and because patterning was complex, reflecting four processes, each operating at a different scale: physiography, human land ownership (259—ha scale), history of catastrophic fires, and history of small—scale fires and timber harvesting.