Habitat associations of cavity-nesting owls in the Sierra Nevada

Abstract

Several species of small, cavity-nesting owls occur in the Sierra Nevada, including in areas impacted by human activities. The owls typically use standing dead trees (snags) for nest sites. Although descriptive studies exist regarding habitats associations around nest and roost sites, few studies have examined habitat associations at larger spatial scales or relative to certain snag characteristics (e.g., density, decay class). To improve our understanding of the habitat associations of these owls, I compared habitat characteristics at 2 spatial scales around areas of owl detection and non-detection. I also examined distances between conspecifics and heterospecifics to determine if clustering of conspecifics or avoidance of predators occurred. I conducted owl broadcast surveys and snag sampling during the spring and summer of 2006 and 2007 in the Lake Tahoe Basin of central Sierra Nevada. I measured additional habitat variables (e.g., vegetation cover, distance to roadways) from pre-existing geographical information system layers. I used stepwise logistic regression to ascertain which variables were influential in predicting owl occurrence. The northern saw-whet owl (Aegolius acadicus) was the only species detected in sufficient numbers for statistical analysis, with a detection probability of 0.25. I detected saw-whets in a wide range of conditions and it appeared that few factors influenced their distribution in the basin. Areas dominated by white fir, however, were correlated with the absence of saw-whets at both the macrohabitat and microhabitat scales. White fir-dominated areas tend to occur on the west side of the basin and it is possible white fir was acting as a proxy for other factors not measured in this study, such as microclimate conditions or prey availability. I was also more likely to find a saw-whet within 1000 m of another saw-whet than within 1000 m of a non-use point, indicating clustering of conspecifics in the basin. While it appears saw-whet needs are being met in the basin, restoration projects are ongoing to decrease both the number of snags and relative abundance of white fir. Continued monitoring of the species is essential to understand potential effects of restoration activities. Suggestions are provided for appropriate timing and effort of future surveys.