Diets of California Spotted Owls in the Sierra National Forest1

From May 1987 through October 1992 and from July through August 1998, we studied diets of California spotted owls (Strix occidentalis occidentalis). Regurgitated pellets were collected at roost and nest sites between 1,000 and 7,600 ft elevation in the Sierra National Forest and were examined for remnant bones, feathers, and insect exoskeletons. Remains of 2,038 individual prey were identified in 1,140 pellets. Woodrats (Neotoma spp.) were the predominant prey in low-elevation oak woodlands and riparian-deciduous forests, accounting for 74.3 percent of the biomass in diets during the breeding period and 81.9 percent during the nonbreeding period. In coniferous forests, northern flying squirrels (Glaucomys sabrinus) were the major prey, comprising 45.6 percent and 77.3 percent of prey biomass during the breeding and nonbreeding periods, respectively. Pocket gophers (Thomomys spp.) were the second largest component of owl diets, by biomass, in both vegetation types. In the breeding period, birds were a larger part of the owl's diet in coniferous forests (12.9 percent) than in the riparian-deciduous and oak habitats (1.8 percent). Other small mammals, insects, and lizards were also found in pellets. Diets differed among years. Concern over the status of the California spotted owl (Strix occidentalis occidentalis) increased when the U. S. Fish and Wildlife Service listed the northern spotted owl (S. o. caurina) and then the Mexican spotted owl (S. o. lucida) as "threatened." Although not currently federally listed, the California spotted owl is classified as a "sensitive species" by the Pacific Southwest Region (Region 5) of the USDA Forest Service. Several studies have examined diets of California spotted owls in the Sierra Nevada (Kadoch 1997, Laymon 1988, Marshall 1942, Thrailkill and Bias 1989, Verner and others 1992), but additional information on diets is needed to better understand the relations between owl occurrence and breeding and the availability of prey. An adequate understanding of diets can then guide studies of the ecological requirements of key prey species and help to guide forest management in ways that will favor conditions for the prey species and, in turn, for the owls. We studied California spotted owl diets in the southern Sierra Nevada to describe prey composition and to examine annual and seasonal variations in the diets. We also compared diets between owls in coniferous forests and owls in oak woodlands and riparian-deciduous forests.