Density of northern spotted owls in northwest California

We estimated densities of northern spotted owls (Strix occidentalis caurina) over 4 years using intensive surveys of marked individuals on a 292-km2 study area in northwest California. For the 4-year sampling period, estimates from empirical and Jolly-Seber methods were in close agreement, yielding a crude density of 0.235 owls/km2 (95% CI = 0.214-0.256) and ecological densities of 0.544 (95% CI = 0.495-0.592) and 0.660 (95% CI = 0.601-0.719) owls/km2 of suitable habitat, using 2 definitions of suitable habitat. Increases in density over the 4 years were attributed to population processes, such as immigration, rather than internal increases within the sampled area. Densities in our area were relatively high when compared with other populations. Our study design should be useful for making geographic and habitat comparisons and for monitoring spotted owl populations. Estimates from our study and the U.S. Forest Service indicate a 60.0-82.5% reduction of current spotted owl populations on at least 1 National Forest under current management plans by the U.S. Forest Service. J. WILDL. MANAGE. 54(1):1-10 Over the past decade, the northern spotted owl has become the focus of controversial management policies in the Pacific Northwest because of the intimate association of the spotted owl with old-growth coniferous forests (Forsman et al. 1984, Gutierrez and Carey 1985, Dawson et al. 1987, Simberloff 1987, Thomas et al. 1988). Thus, an important element in their management will be estimating and monitoring changes in abundance of spotted owls (Dawson et al. 1987). Animal abundance can be measured as the number of individuals in the population (population size), the number of individuals per unit area (absolute density), or the density of 1 population relative to another (relative density) (Caughley 1977:12). Absolute density can be further subdivided into crude density-measured with respect to all of the area containing the population, and ecological density-measured with respect to a particular habitat (Johnson 1978:11, Tanner 1978:2). Absolute density is necessary for determining population and trophic dynamics (Verner 1985) and effective population sizes of demes (Barrowclough and Coats 1985). Relative density has been used to make comparisons between spotted owl populations and habitats (Forsman et al. 1977, Garcia 1979, Marcot and Gardetto 1980). However, these measures require standardization by some measure of absolute density (Caughley 1977:14, Verner 1985) and similar detection probabilities of individuals (Andersen et al. 1985) to ensure that relative measures provide an accurate index of abundance. Therefore, absolute density provides an initial step in estimating abundance. In this paper, we report empirical and markrecapture estimates of absolute density of northern spotted owls in northwestern California from 1985 through 1988. Our objectives were to estimate absolute density of northern spotted owls, to provide general guidelines for estimating density of northern spotted owls, and to evaluate our estimates in relation to current management plans. Our study would not have been possible without the field assistance of K. E. Young, C. A. Moen, J. A. Blakesley, T. J. Evans, and M. B. Kasper. C. A. Moen and J. A. Blakesley also assisted in data compilation. We would like to thank D. M. Solis, J. Kahl, D. Kudrna, J. L. Mattison, L. Roberts, A. C. Chrisney, M. Fox, and M. Smith of the Six Rivers National Forest for their help in providing information during this study. R. Rhino provided transportation. G. F. Barrowclough, J. A. Blakesley, J. A. Crawford, E. D. Forsman, W. S. LaHaye, J. D. Nichols, B. R. Noon, D. M. Solis, N. G. Tighlman, and J. Verner reviewed earlier drafts of this manuscript. Funding was provided by California Department of Fish and Game, Federal Aid