Diet and Home-range Size of California Spotted Owls in a Burned Forest

Fire is pervasive in forests used by California Spotted Owls (Strix occidentalis occidentalis) and their prey species. We assessed the diets and sizes of the breeding-season home ranges of seven Spotted Owls occupying burned forests in the southern Sierra Nevada 4 years after a fire and compared the results with data from previous studies in unburned forests within the range of the subspecies. Prey captured by owls in the burned area comprised 40.3% (by biomass) pocket gophers (Thomomys spp.) and 25.9% northern flying squirrels (Glaucomys sabrinus). In contrast, in unburned areas of the Sierra Nevada Spotted Owls fed primarily on flying squirrels, or on both flying squirrels and woodrats (Neotoma spp); in unburned southern California forests they fed overwhelmingly on woodrats. The owls’ mean home range in the burned forest covered 402 ha, an area similar to that recorded in unburned forests of the Sierra Nevada. Our results are consistent with hypotheses that the burned habitat in our study area was rich in gophers and that Spotted Owls foraging on gophers in burned forests do not require home ranges substantially larger than do owls in unburned forests. With currently available data we could not conclusively attribute variation in diet or home-range size to the influence of fire, so further testing is warranted. Use of rodenticides and herbicides in managing burned Spotted Owl habitat may reduce the owl’s key prey. The California Spotted Owl (Strix occidentalis occidentalis) occurs in mixed-conifer and montane hardwood forests throughout the Sierra Nevada and the mountain ranges of southern California (Verner et al. 1992, Gutiérrez et al. 1995). Its vital rates are positively associated with large areas of older conifer-dominated forest and related conditions (Blakesley et al. 2005, Seamans 2005), and within its range the subspecies is viewed as an indicator for management of these forest types. Forest fire is one of the most important issues affecting the Spotted Owl’s habitat (Weatherspoon et al. 1992). California Spotted Owls typically occupy older forests with greater canopy cover (Bias and Gutiérrez 1992, Call et al. 1992, Gutiérrez et al. 1992, Moen and Gutiérrez 1997, Bond et al. 2004), and this has led to the presumption that fires of moderate and high severity reduce the owl’s survival and pose a significant risk to its habitat (Weatherspoon et al. 1992, U.S. Forest Service 2001, 2004, 2005). However, Spotted Owls can occupy territories and continue to reproduce in burned habitats, including those with large severely burned patches (Gaines et al. 1997, Bond et al. 2002, Jenness et al. 2004, Bond et al. 2009). Franklin et al. (2000) hypothesized that fire could enhance the abundance of and access to prey for the Northern Spotted Owl (S. o. caurina) in California by creating patchy openings in the forest canopy and increasing habitat edges. Indeed, four years after a large, patchy fire in the southern Sierra Nevada, California Spotted Owls selectively foraged in burned forest rather than unburned forest, with the strongest selection for severely burned patches (Bond et al. 2009). CALIFORNIA SPOTTED OWLS IN A BURNED FOREST 115 Prey composition is an important determinant of raptors’ use of space (Peery 2000). Zabel et al. (1995) found that prey species predicted the size of the Northern Spotted Owl’s home range in California better than the proportion of older forest in the home range, and Carey et al. (1992) reported that the prey base strongly influenced the size of the Northern Spotted Owl’s home range in Oregon. In both studies, owls with a greater percent biomass of larger prey in their diet had home ranges smaller than those of owls that consumed more of the smaller prey species. Ward et al. (1998) documented that in California Northern Spotted Owls selected foraging sites according to the distribution of larger prey (e.g., woodrats), which provided an energetic benefit to the owls. The California Spotted Owl’s major prey items by biomass are the dusky-footed and big-eared woodrats (Neotoma fuscipes and N. macrotis), northern flying squirrel (Glaucomys sabrinus), and pocket gophers (Thomomys spp.; Thraikill and Bias 1989, Williams et al. 1992, Smith et al. 1999, Munton et al. 2002). Despite the importance of fire in shaping ecosystems in portions of the Spotted Owl’s range, little is known about the diets and home-range sizes of owls occupying recently burned forests. Understanding the California Spotted Owl’s prey base and space use in burned forests is important because fire is frequent in this subspecies’ habitat, and it is not unusual for owls to continue to occupy and breed in burned forests (Bond et al. 2002, 2009, Roberts et al. 2011). Furthermore, forest management after a fire can include removal of fire-killed trees and applications of rodenticide and herbicide in efforts to enhance the growth of tree seedlings (McGinnis et al. 2010, Swanson et al. 2011), possibly to the detriment of foraging Spotted Owls and their prey. Our objective was to assess the breeding-season diet and estimate the sizes of the home ranges of California Spotted Owls occupying territories in recently burned mixed-conifer forests and compare the results with those of previous studies of owls in unburned forests. We note that the breeding-season home range may differ greatly from the year-round home range, as we have reported previously on the expanded winter movements of the same owls addressed in this study (Bond et al. 2010). Our sample of owls in burned forests was relatively small (seven owls from four territories), our study included only a single breeding season, and the data from unburned forests we used for comparison were recorded in different years and in different areas. Therefore, rather than testing how fire affected diets and home-range sizes, we report for the first time basic information about the diet and home-range sizes of the California Spotted Owl in a burned forest and address differences from those in unburned forests qualitatively—a basis for developing hypotheses about fire effects to be tested in future studies.