|Abstract or Summary
- Spatial variation in the diet of the northern spotted owl (Strix occidentalis caurina) influences owl home ranges, and understanding this relationship will aid managers in forest management prescriptions that influence spotted owl recovery. This study describes the spatial variation in owl diet based on 4183 prey collected at 114 owl territories in 1833 km2 area of the Central West Cascades during 1988-2009. The study addressed two questions: (1) What are the spatial patterns of owl territories and prey in owl pellets? (2) What landscape characteristics explain the composition of spotted owl prey? Thirteen prey species/groups were identified as key prey to spotted owl diets because they appeared in 10% of owl territories, comprised 90% of the total abundance and 95% of the total biomass across all pellet samples. Northern flying squirrels (Glaucomys sabrinus) were the most important prey in all areas, comprising 46-64% of prey abundance and 48-75% of prey biomass. The spatial distribution of key prey species, especially red tree voles (Arborimus longicaudus), pocket gophers (Thomomys mazama), and rabbits/hares (Sylvilagus bachmani /Lepus americanus) was significantly related to easting, elevation, and fine-scale relief. Red tree voles were more abundant in owl diets at low elevation, with high fine-scale relief and in the western portions of the study area, whereas pocket gophers and rabbits/hares were more abundant in owl diets at high elevation, with low fine-scale relief and in the eastern portions of the study area. Owl territories exhibited a significantly dispersed spatial pattern in almost all years, but the mean nearest neighbor distance between owl territories was 2090m in the western and 3000m in the eastern portions of the study area. Differences in owl pair densities and nearest neighbor distances were related to spatial patterns in owl diet. Where red tree voles comprised a higher proportion of the diet, owl pair density was higher and owl territories were more closely packed than where pocket gophers predominated. These findings suggest that (1) differences in diet among local areas was due to differences in key prey availability, which in turn are influenced by vegetation zone and topography, (2) at high elevation spotted owl sites with abundant pocket gophers and few woodrats in the diet, owl dietary dependence on flying squirrels over the winter and early in the nesting period may require owls to occupy larger territories than owls require in lower elevations, and (3) the spatial variation in owl diets and associated variation in owl pair densities and nearest neighbor distances suggest that owl habitat requirements vary within the West Cascades physiographic province, raising questions about the use of one-size-fits-all habitat values as a management strategy. These findings may be relevant for regulatory agencies and forest managers as they work to recover the spotted owl.