Diversity of great gray owl nest sites and nesting habitats in California

摘要

The great gray owl (Strix nebulosa) is listed by the state of California as endangered, with a population estimate of fewer than 300 individuals in the state. Nest-site availability has been suggested as a limiting factor for population growth in California, but information on nest types and nesting habitat has been based on a small number of nests that may not fully represent the variety of conditions used by the species. We collated all known nesting records in the Sierra Nevada mountains of California since 1973 (n=56) and then visited 47 of the nest sites to characterize habitat and compare them with paired reference sites. Great gray owls used a diversity of trees (8 species) and nest types. Although great gray owls in California are considered conifer-forest specialists, 30% of nests were in oak trees and 21% were below 1,000m, which loosely corresponds to the lower conifer-zone limit. Across all elevations and tree species, degree of deterioration was the most important factor differentiating nest trees from paired reference trees at the same meadow, with nest trees being significantly more decayed. Nest trees (mean dbh=100.5 +/- SD 30.3cm) were also significantly larger than reference trees. Canopy cover within 50m of nest trees ((x) over bar =85.1 +/- 16.4%) was significantly greater at nest sites than at reference sites. At higher elevations, most nests were within 250m of a meadow edge, but at lower elevations, 31% of nests were >750m from the closest meadow. Based on these findings, we suggest that managers trying to promote great gray owl nesting maintain 4 or more large (100-cm dbh) snags per hectare in dense forests, especially near meadows. We also recommend increasing great gray owl survey effort in habitats and areas that may have been inadequately surveyed in the past. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.