Demographic Characteristics and Trends of Northern Spotted Owl Populations in Northwestern California

In California, research on the distribution and numbers of Northern Spotted Owls (Strix occidentalis caurina) began in the early 1970’s (Gould 1977). Early indications that the owl population could be declining in California received little attention until the early 1980’s when focused ecological research on the owl began in northwestern California (Gutiitrrez et al. 1984). Debate over trends in Northern Spotted Owl populations in California recently culminated in a petition to remove the owl in California from the federal list of threatened and endangered species (California Forestrv Association 1993). Much of this recent debate has centered around predictive modeling approaches which lacked supporting empirical data (Harrison et al. 1993). In 1985, we began a long-term study on the population ecologv of the Northern Spotted Owl in northwestern California (Franklin et al. 1990). Our study followed five years of previous studies in the same region that established baseline information on Spotted Owl habitat and dispersal ecologv (Guti&rez et al. 1985, LaHaye 1988, Sisco 1990, Solis and Gutikrrez 1990). Objectives of our long-term demographic research have been to document life history characteristics of Northern Spotted Owls, monitor long-term population trends on public lands in northwestern California, and to relate estimates of fitness with measures of habitat structure and composition. In this paper, we present estimates of ageand sex-specific survival probabilities, fecundity rates, and trends in those estimates over a 9-year period. In addition, we test the null hypothesis that the population of Spotted Owls in northwest California was stationary or increasing against the alternative that the population was declining during the period of our study. Throughout this paper, we use the term stationary when referring to constant population size over time and stable when referring to constant demographic parameters over time (Seber 1982:400).