A study of the ecology of the spotted owl (Strix occidentalis) was conducted on the west slope of the Sierra Nevada, California from June 1982 to January 1985. Nine juvenile and 12 adult spotted owls were captured and fitted with radio transmitters. Tests of survey techniques showed an average response rate of 25% to nocturnal call surveys. Males responded more frequently than females and non-breeding owls responded more frequently than breeding owls. Reproductive success was low, with only 5 of 50 pairings producing young. Juvenile survival prior to dispersal (n = 12), was also low (33%). These results predict a rapidly declining population if not supplemented by immigration. Home ranges were determined by the minimum convex polygon method. The median summer-fall home range size for an individual spotted owl was 1210 ha (2988 acres); the median summer-fall home range size for a pair was 1660 ha (4100 acres). In November, the owls migrated downslope from mixed conifer forest above 1300 m (4265 feet) to oak-pine woodlands below 1300 m. Despite occupancy of the study area by spotted owls the site is believed to be marginal habitat based on the poor reproductive success and high mortality rates observed during the study. -- Adapted from abstract.