Owl vs. Owl

Natural History, March, 1999 by Sharon Levy

A new twist emerges in the turf war over Pacific Northwest forests as a new adversary invades the remaining haunts of the threatened spotted owl.

Just before dawn, a chill fog drifts through the old-growth redwoods of northwestern California. A group of birders breathe out puffs of steam as they listen to the growing chorus of morning birdsong. Then the gentle sounds of kinglets and thrushes are buried under a torrent of avian rock 'n' roll as the wild, intense hoots of a barred owl ring out. It is one of the first recorded sightings of this species in this part of California. A couple of months later, in May 1997, an agitated barred owl will be found perched near the body of a freshly killed spotted oval in Redwood National Park, near the Oregon border, feathers of his presumed victim stuck in his talons. The latest turf war in the Pacific Northwest has reached redwood country.

Dark-eyed woodland species, the barred owl and spotted owl are cousins that look so similar that novice birders have trouble telling them apart. Until recently, the two birds never met. The barred owl haunted forests east of the Great Plains, while the spotted owl lived only in old conifer forests of the Pacific Northwest. Now the barred owl is on the move--and it is moving in on the threatened spotted owl.

"My educated guess is that the barred owl will have a dramatic effect on the spotted owl," says Eric Forsman, a biologist with the U.S. Forest Service's Pacific Northwest Research Station in Corvallis, Oregon. Twenty years ago, Forsman's research on the northern spotted owl alerted conservationists to the bird's dependence on mature forests, which had Been heavily logged for decades, depleting much of the bird's habitat. This information helped lead to the owl's listing as a threatened species under the federal Endangered Species Act. More recently, Forsman and his colleagues have also been documenting the invasion of the barred owl into the Pacific Northwest.

"For the last thirty years we've been trying to come up with ways of protecting the spotted owl," says Forsman, "and now all of a sudden, this huge monkey wrench gets thrown into the works. In the past, we could assume that what we were seeing in terms of habitat would help us to understand what was happening with the spotted owl. Now we don't know if spotted owls aren't there because there is no habitat for them or because of the barred owls."

"The barred owl is a generalist, so it'll eat almost anything," says Tom Hamer, a consulting biologist who has studied the interaction of the two owls in the northern Cascade Range of Washington State. "It will eat flying squirrels and snowshoe hares, which the spotted owl also eats. But the barred owl will also hunt trout and amphibians in small streams and eat anything else that crosses its path, including grouse." Because the barred owl is such an adaptable hunter, it can live off a home range of only about 1,600 acres in the northern Cascades. But spotted owls of this region are specialists, taking mostly arboreal mammals like flying squirrels and red tree voles. To find enough food to survive, spotted owls need large areas of the older forests that support their prey. In the redwood region (from the Oregon border south to San Francisco Bay), spotted owl home ranges are generally small, because wood rats provide an abundant food source, but in the Cascades, ranges can span 30,900 acres.

During territorial disputes, the owls will hoot at each other (their calls are similar, but the spotted's has a slower, hesitant tempo) and fly at each other, but actual combat seems to be rare. "Barred owls always win," says Hamer. "When it comes to territorial competition, the spotted owl always backs off. Part of it is body size--the barred owl is about 20 percent heavier--but another part of it is behavior. The spotted owl just isn't very aggressive."

How did the barred owl end up on the beleaguered spotted owl's turf? "It's difficult to understand the failure of barred owls to colonize western North America prior to recent times without invoking human influence," says Rocky Gutierrez, a professor at Humboldt State University in Arcata, California. He believes that human tinkering with the western landscape made the barred owl's range expansion possible. As Euro-American settlers colonized the Great Plains, they planted shelter belts of trees across the Midwest and suppressed the natural fires that had kept the grasslands open. The result was a patchwork of small woodlands that may have served as stepping stones across the prairies for the barred owl, as they did for many birds formerly restricted to forest edges throughout eastern North America. One 1983 study of birds in western Minnesota found that only three of the forty-seven most abundant species using forest shelter belts were typical of the historic grasslands there.

Closely related species--such as the eastern and western screech owls--have not only met but have also mated on the plains. (Such hybridization is fairly common and points up the fact that natural populations are always in flux and can defy human efforts to classify them.) Some barred owls, too, started to form pairs with their spotted cousins as soon as the ranges of the two began to overlap in the Pacific Northwest. But these mixed pairs and their "sparred" offspring are relatively rare. Forsman finds the barred/spotted pairs producing hybrid young in his study region in Oregon intriguing, but he notes that when barred owls meet spotted owls, a takeover rather than a partnership is far more likely: "Barred owls move in, and spotted owls get darned hard to find or move out of the area."