Northern Spotted Owl has long been the most famous bird in the United States.
Listed in June 1990 as Threatened under the Endangered Species Act because of declines in old-growth forests throughout its range, it became the centerpiece of a bitter environmental battle that was settled in 1994, when annual timber harvests in the Pacific Northwest were dramatically reduced on its behalf.
Then, in the summer of 2013, it became the subject of controversy again, as the U.S. Fish and Wildlife Service set in motion a plan to shoot Barred Owls on federally managed land in California, Oregon, and Washington in order to determine if Spotted Owls would come back if their chief competitors were removed.
Now, from researchers who analyzed data collected from 11 study areas in Washington, Oregon, and northern California between 1985 to 2013, we learn that Northern Spotted Owl populations are shrinking in all parts of their range in the Pacific Northwest. The owl, one of three subspecies of Spotted Owl, is declining nearly four percent per year.
Since monitoring began, say the researchers, Spotted Owl populations fell 55-77 percent in Washington, 31-68 percent in Oregon, and 32-55 percent in California. In addition, population declines are now occurring on study areas in southern Oregon and northern California that were previously experiencing little to no detectable decline through 2009.
A team of 37 investigators collaborated on the study, which was just published in the journal The Condor: Ornithological Applications. We quoted two of them — Dale Herter, a wildlife biologist with Raedeke Associates, and Eric Forsman, of the USDA Forest Service, Pacific Northwest Research Station — in a feature article about Barred and Spotted Owls published in October 2012.
The researchers and their colleagues found no shortage of evidence that Barred Owl is playing a pivotal role in the Spotted Owl’s continued decline, although habitat loss and climate variation were also important in some parts of the subspecies’ range.
Barred Owl was once known to occur only as far west as the edge of the Great Plains but over the last half century has spread westward. Today it is fairly common along the Pacific Slope from southeastern Alaska to central California — that is, throughout the range of Northern Spotted Owl.
“This study provides strong evidence that Barred Owls are negatively affecting Spotted Owl populations,” says lead author Katie Dugger. “The presence of Barred Owls was associated with decreasing Spotted Owl survival rates in some study areas, and Spotted Owls were disappearing from many of their historical breeding territories as those areas were invaded by Barred Owls.”
The exception was a small study area in northwestern California owned by the Green Diamond Resource Company, where Barred Owl removals began in 2009, and where long-term population declines were only nine percent. There, Spotted Owl populations and survival rates have increased since the removal of Barred Owls started.
“The removal of Barred Owls from the Green Diamond Resources study area,” write the researchers, “had rapid, positive effects on Northern Spotted Owl survival and the rate of population change, supporting the hypothesis that, along with habitat conservation and management, Barred Owl removal may be able to slow or reverse Northern Spotted Owl population declines on at least a localized scale.”
The investigators stress that further research is required on the effects of removing Barred Owls in other parts of the Spotted Owl’s range — especially in Washington, where Barred Owl numbers have been high for a long time.
Adds Dugger, a research biologist at the USGS Oregon Cooperative Fish and Wildlife Research Unit at Oregon State University: “The amount of suitable habitat required by Spotted Owls for nesting and roosting is important because Spotted Owl survival, colonization of empty territories, and number of young produced tend to be higher in areas with larger amounts of suitable habitat, at least on some study areas.”
Relationships between Spotted Owl populations and climate were complex and variable, but the study suggests that survival of young Spotted Owls and their ability to become part of the breeding population increase when winters are drier. This may become a factor in population numbers in the future, given climate-change predictions for the Pacific Northwest include warmer, wetter winters.
Read the paper
Katie M. Dugger, Eric D. Forsman, Alan B. Franklin, Raymond J. Davis, Gary C. White, Carl J. Schwarz, Kenneth P. Burnham, James D. Nichols, James E. Hines, Charles B. Yackulic, Paul F. Doherty, Jr., Larissa Bailey, Darren A. Clark, Steven H. Ackers, Lawrence S. Andrews, Benjamin Augustine, Brian L. Biswell, Jennifer Blakesley, Peter C. Carlson, Matthew J. Clement, Lowell V. Diller, Elizabeth M. Glenn, Adam Green, Scott A. Gremel, Dale R. Herter, J. Mark Higley, Jeremy Hobson, Rob B. Horn, Kathryn P. Huyvaert, Christopher McCafferty, Trent McDonald, Kevin McDonnell, Gail S. Olson, Janice A. Reid, Jeremy Rockweit, Viviana Ruiz, Jessica Saenz, and Stan G. Sovern (2016) The Effects of Habitat, Climate, and Barred Owls on Long-Term Demography of Northern Spotted Owls. The Condor: Ornithological Applications, February 2016, Vol. 118, No. 1, pp. 57-116.
Updated: Working links to research paper added and citation updated, December 11, 2015.
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