An update of demographic estimates for the Northern Spotted Owls (Strix occidentalis caurina) from Oregon's Central Coast Ranges

Abstract

Demographic characteristics of the Northern Spotted Owl (Strix occidentalis caurina) were studied on the Eugene District Bureau of Land Management, central Oregon Coast Ranges from 1989-1995. Survival rates were estimated from capture histories of banded owls using Cormack-Jolly-Seber open population models. We banded 233 owls, including 119 that were ≥ 3 years old, 15 that were 1 or 2 years old, and 99 juveniles. Among year variation in the proportion of pairs nesting and fecundity of females was significant ( P < 0.001). Estimates of apparent annual survival from the selected capture-recapture models were 0.306 (SE = 0.064) for juveniles and 0.875 (SE = 0.018) for subadult and adult owls combined. The estimated annual rate of population change (0.939, SE = 0.045) was < 1.00 (P = 0.005) over the 6 years of study, suggesting an average population decline of 6.1 per cent per year. Counts of territorial owls decreased by 37 percent from 1990-1995 on the Wolf Creek density study area, a smaller area within the larger surrounding study area. We suggest the owl population decline was due to the reduction of spotted owl habitat.

Parent Publication

• Biology and conservation of owls of the Northern Hemisphere: 2nd International symposium

Citation

Thrailkill, James A.; Anthony, Robert G.; Meslow, E. Charles. 1997. An update of demographic estimates for the Northern Spotted Owls (Strix occidentalis caurina) from Oregon''s Central Coast Ranges. In: Duncan, James R.; Johnson, David H.; Nicholls, Thomas H., eds. Biology and conservation of owls of the Northern Hemisphere: 2nd International symposium. Gen. Tech. Rep. NC-190. St. Paul, MN: U.S. Dept. of Agriculture, Forest Service, North Central Forest Experiment Station. 432-448.