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

Abstract

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 1,000 m, 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 30.3 cm) were also significantly larger than reference trees. Canopy cover within 50 m of nest trees (x¯ = 85.1 ± 16.4%) was significantly greater at nest sites than at reference sites. At higher elevations, most nests were within 250 m of a meadow edge, but at lower elevations, 31% of nests were >750 m 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.

Great Gray Owls Nesting in Atypical, Low-Elevation Habitat in the Sierra Nevada, California

“…Great Gray Owl nests were typically surrounded by dense forest (.60% canopy closure), consistent with findings from elsewhere in California (Greene 1995, Wu et al 2015, Oregon (Bull and Henjum 1990), and eastern Idaho and northwest Wyoming (Whitfield and Gaffney 1997). More surprising was that the forests in a 900-m radius around nests contained large areas (109-204 ha) dominated by small-diameter trees (25.4-50.6 cm), whereas other research in California, based primarily on mid-elevation nest sites, has emphasized the importance of large trees in mid-to late-seral nest stands (Winter 1986, Sears 2006, Wu et al 2015. In northeastern Oregon, Bull and Henjum (1990) found Great Gray Owls nesting in large trees in relatively late-seral stands, whereas in southwestern Oregon, areas around Great Gray Owl nests were characterized by a high number of small trees and low densities of large trees (Fetz et al 2003), similar to what we observed at our study site.…”

“…Half of the nesting territories had 70-89% conifer cover and 8-30% hardwood cover, whereas the remaining sites had 8-60% conifer and 20-71% hardwood cover. The average percent cover of hardwood across the eight sites in our study area was higher than around Great Gray Owl nests elsewhere in California (Wu et al 2015), reflecting the overall greater abundance of oak trees in lower-montane forests of the Sierra Nevada than at higher elevations. Great Gray Owl nests were typically surrounded by dense forest (.60% canopy closure), consistent with findings from elsewhere in California (Greene 1995, Wu et al 2015, Oregon (Bull and Henjum 1990), and eastern Idaho and northwest Wyoming (Whitfield and Gaffney 1997).…”

“…The average percent cover of hardwood across the eight sites in our study area was higher than around Great Gray Owl nests elsewhere in California (Wu et al 2015), reflecting the overall greater abundance of oak trees in lower-montane forests of the Sierra Nevada than at higher elevations. Great Gray Owl nests were typically surrounded by dense forest (.60% canopy closure), consistent with findings from elsewhere in California (Greene 1995, Wu et al 2015, Oregon (Bull and Henjum 1990), and eastern Idaho and northwest Wyoming (Whitfield and Gaffney 1997). More surprising was that the forests in a 900-m radius around nests contained large areas (109-204 ha) dominated by small-diameter trees (25.4-50.6 cm), whereas other research in California, based primarily on mid-elevation nest sites, has emphasized the importance of large trees in mid-to late-seral nest stands (Winter 1986, Sears 2006, Wu et al 2015.…”

“…A recent compilation of all known nesting records in California (including the nests described here) suggested that the lower-montane zone, defined here as the transition zone between oak-dominated woodlands and conifer-dominated forests, may be more important for California's Great Gray Owl population than previously thought, as 21% of 56 documented nest sites were below 1000 m (Wu et al 2015). However, it remains unclear whether nests in the lower-montane zone produce fledglings at similar rates as nests in more typical habitat for Great Gray Owls in California (i.e., mid-elevation forests adjacent to montane meadows), or instead if the zone might function as a sink for the species.…”

“…Although our sample size is small, Great Gray Owl nests are rarely found in California (only 56 nests found since 1973; Wu et al 2015) and little is known of their reproductive output. We also describe habitat characteristics around the nests and use the information to develop a predictive landscape-level model for identifying areas with similar characteristics across the lower-montane zone of the Sierra Nevada.…”