Old-Forest Distribution around Spotted Owl Nests in the Central Cascade Mountains, Oregon

Abstract

Unlike previous spotted owl (Strix occidentalis) habitat association studies, we restricted our inquiry to the old-forest type and thus explored the association of spotted owls with habitat distribution as opposed to habitat type. We compared old-forest distribution around 126 northern spotted owl (S. o. caurina) nests in 70 pair territories, 14 nonreproductive spotted owl activity centers, and 104 points drawn randomly from old forest (closed canopy, >80 yr) in the central Cascade Mountains of Oregon. We quantified the percentage of old forest within 50 concentric circular plots (0.1-5.0-km radii) centered on each analyzed point, and we used logistic regression to make spatially explicit inferences. Owl nests were surrounded by more old forest in plots with 0.2-0.8-km radii (P < 0.05). Results suggested the landscape scales most pertinent to northern spotted owl nest-site positioning in this study were (in descending order) (1) the surrounding 15 ha (approx 200-m radius), (2) the surrounding 30-115 ha (approx 300-600-m radius), (3) the surrounding 200 ha (800-m radius), and (4) possibly the surrounding 700 ha (1,500-m radius). Nests were associated with higher proportions of old forest near the nest, implying that the arrangement of habitat was important for nest-site selection, posi- tioning, or both. The 70 territories of nesting owls had more old forest on average than did the 14 nonrepro- ductive owl sites, and the probability that a pair nested at least once during the study was positively associated with area of old-forest habitat in all radii studied. Because spotted owls in the central Cascade Mountains of Oregon are known to have home ranges that average 1,769 ha, our results apply to nest-site location on the landscape and not to the amount of habitat necessary for pair persistence or successful reproduction.