SPOTTED OWL HOME-RANGE AND HABITAT USE IN YOUNG FORESTS OF WESTERN OREGON

Abstract

To assess spotted owl use of young forests, we studied home-range sizes and habitat-use patterns of 24 adult northern spotted owls (Strix occidentalis caurina) on 2 sites in the Oregon Coast Range: the Elliott State Forest (ESF) and state forest lands in the Northern Coast Range (NCR). Conifer forests at ESF were characterized by a mixture of old, mature, and pole-sized conifer, similar to other areas occupied by spotted owls in western Oregon, USA. In contrast, conifer forests at NCR were younger than most other sites occupied by spotted owls in western Oregon and consisted primarily of conifers <80 years old. Broadleaf forest also was abundant (approx 22%) at both ESF and NCR. We used an information–theoretic approach and Akaike's Information Criterion (AIC) to evaluate a priori hypotheses about spotted owl home-range sizes and habitat-use patterns on our study areas. Considering previous knowledge about habitat requirements of the species, we predicted that owls occupying sites with fewer old conifer stands would have larger home ranges and that owls would select the oldest and most structurally diverse forest available for foraging and roosting. Our top model for evaluating home-range sizes indicated that the proportion of older conifer forest within the home range best explained the variability in home-range sizes. Although we found considerable variation in home-range size among owls, home-range sizes at ESF generally were smaller than home-range sizes at NCR, and home ranges at both sites were smaller than those reported for other study areas in western Oregon. Habitat-use patterns also varied widely among owls both within and between sites. Models containing distance to the nest tree, proximity to nearest forest edge, and proximity to nearest broadleaf-forest edge were the most parsimonious models for distinguishing owl locations from random points. On average, owl locations at both study areas were closer to ecotones between broadleaf forest and other cover types and farther from forest–nonforest ecotones than random points. Overall, we did not observe strong selection or avoidance of any cover type, although owls at ESF showed greatest use of older conifer forest while owls at NCR showed greatest use of broadleaf forest. Use of these habitat configurations and cover types by spotted owls had not been well documented prior to our study. The predictive power of our models was not great, however, indicating that factors in addition to those we included in our analysis may have influenced owl habitat-use patterns at our study areas. Based on our results, we recommend that managers at these sites maintain existing old and mature conifer forest, broadleaf forest, broadleaf-forest edges, and forested riparian areas as owl habitat; avoid timber harvest in core use areas; and plan the size of areas managed for spotted owls to reflect actual home-range and core-area sizes for owls in those forests.