CORVALLIS, Ore. - The total number of spotted owls in Washington, Oregon and northern California appears to be declining about 4 percent a year, but the survival rate for adult females is steady so there is still uncertainty about the bird's overall health.
That is the conclusion of a new report compiled by about 50 federal, state, university and private scientists who met on the Oregon State University campus in December to analyze demographic data collected at 16 sites in the three states.
The northern spotted owl was added to the federal endangered species list in 1990, which led to cutbacks in western timber harvests in certain types of older forests, and to changes in other land management practices.
The new report, titled "Range-wide Status and Trends in Northern Spotted Owl Populations," looks at data collected between 1985 and 1998. It is a follow-up to a similar analysis conducted in 1993.
Such analyses of data regularly collected at designated nest sites are part of the requirements of the Northwest Spotted Owl Effectiveness Monitoring Plan for the federal Northwest Forest Plan. The sampling sites are mostly on federal land, although portions of some are on private and state land.
Key findings in the new analysis:
- There was a 4 percent average annual decline in the abundance of territorial females, says the report, noting that some individual study areas had stationary population growth while others had substantial declines.
- The survival rate of adult females combined for all study areas did not show a decline. This differed from the 1993 analysis, which found adult female survival rates to be declining. "We do not know if this change is due simply to adding more years of data to the sample," the report states, "or reflects a response to reduction in harvest rates on federal lands, or is due to some other environmental factor(s)."
- "The mean number of young produced per female varied among years and did not exhibit any linear time trends," according to the report. "Years of high and low reproduction tended to occur in alternate years....This result is basically the same as was noted in ... 1993."
This result "indicates that productivity (of young spotted owls) has been stable over the years of the study," said Robert Anthony, leader of the Oregon Cooperative Fish and Wildlife Research Unit, based at Oregon State University's Department of Fisheries and Wildlife.
Overall, concludes the report's executive summary, "there is still uncertainty regarding the health of the spotted owl population."
The report notes that scientists are exploring several approaches that may help in the future in determining the bird's population trends.
Senior author of the report was Alan Franklin, a wildlife biologist with the Colorado Cooperative Fish and Wildlife Research Unit, a division of the U.S. Geological Survey's Biological Resources Division.
Other authors included Anthony and Eric Foresman of the U.S. Forest Service Forest Sciences Lab, Corvallis. Ore.