Spotted owls are declining thanks to an owl interloper

Perhaps no animal is more symbolic of the North American conservation movement and its struggles with business interests as the spotted owl. In 1990, the northern spotted owl (Strix occidentalis caurina) was listed as a threatened subspecies under the Endangered Species Act. Four years later the Northwest Forest Plan was implemented—an effort intended to conserve all native species in the Pacific Northwest including spotted owls.

The white-spotted brown owls have made their homes in the old growth forests of the area for hundreds of years, nesting in tree cavities and feasting on the bugs drawn to decaying wood. Over the last century and a half, though, those forests have slowly been logged. Cedars and spruces don’t just make a fine home for a pair of owls; they also make for beautiful handmade furniture. As the forests shrunk, so too did spotted owl populations. Environmentalists argue that whatever habitat remains ought to be protected for the wildlife that relies on it for its survival; the logging industry says that preserving the landscape comes at the cost of depriving loggers of work.

As part of ongoing efforts to gauge the efficacy of the Northwest Forest Plan, a handful of federal agencies have worked together to monitor the wildlife there, including the northern spotted owl. Eight study areas on federal land were highlighted for the sub-species. In addition, other researchers studying owl populations on private and tribal lands contributed their data, making for a total of 11 study areas: three in Washington, five in Oregon, and three in California. The collection efforts at each site range from 22 to 29 years.

A consortium of researchers from the USGS, USDA, USDI, USFWS, Colorado State University, Simon Fraser University, Oregon State University, Virginia Tech, the Washington Department of Fish and Wildlife, as well as from three private companies and the Hoopa Tribe, discovered that northern spotted owls declined around 4% every year between 1985 and 2013. Since monitoring began, that’s a decline of 55-77% in Washington, 31-68% in Oregon, and 32-55% in California. A few populations in Oregon and California that seemed somehow protected from those declines for most of the study also began to decline starting around 2009.

Those 37 researchers had the aid of nearly 240 field assistants, who helped fit more than 12,000 colored bands onto the legs of nearly 6,000 spotted owls.

The researchers found that habitat loss and climate change were both responsible for at least some of the decline in some areas, but the most important driver of the spotted owls’ decline was the barred owl (Strix varia) invasion.

At one time, the barred owl was restricted to forests in eastern North America, but it’s now found everywhere spotted owls are in the west, and nobody knows what’s allowed them to so dramatically increase the size of their range. Whatever the reason, many wildlife biologists have speculated that their presence has a negative effect on the native spotted owl, as is so often the case for invasive species. That could be, in part, due to competition for resources. In any contest, the larger invaders, who occur at much higher densities, typically dominate the smaller natives. Barred owls enjoy having neighbors, while their spotted conspecifics prefer having a bit more space to stretch their wings.

“This study provides strong evidence that…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,” said USGS and Oregon State University research biologist Katie Dugger in an official statement. Dugger was lead author of the report published this week in The Condor.

Warmer, wetter winters also spelled trouble for spotted owls, as has the earlier arrival of spring. Combined with habitat loss through logging (and, on federal lands, through wildfire), and the conditions are right for localized spotted owl extinctions.

The researchers were unable to determine just how the barred owls disrupted life for the spotted ones. Were spotted owls more likely to die in the presence of barred owls, perhaps because of starvation? Or do barred owls force spotted owls out of the neighborhood, making it tough for them to establish nests and raise their offspring? Evidence from one site suggests it’s the latter.

The observations are only anecdotal, but on land owned by the Green Diamond Resource Company in northern California, spotted owls successfully recolonized territories after barred owls were removed. That suggests, according to the researchers, “displacement from territories into a nonbreeding floater population was at least one mechanism by which apparent survival…[was] negatively affected by barred owls.” In other words, the interlopers didn’t kill off the spotted owls, but they made it difficult for them to ensure the long-term viability of their populations by breeding effectively. On Green Diamond Resource Company land, individual spotted owls were able to reclaim their original territories (or ones just adjacent) after as many as seven years.

“Competition with barred owls may be the primary cause of northern spotted owl population declines across their range,” writes Dugger and her colleagues. Indeed, despite continuing management of federal lands for spotted owl habitat, the long-term viability of the species may be in question unless a barred owl intervention is implemented as well. “Barred owl removal may be able to slow or reverse northern spotted owl population declines on at least a localized scale,” they say. Or: out with the new and in with the old! – Jason G. Goldman | 11 December 2015

Source: Katie M. Dugger, et al. (2016). The effects of habitat, climate, and Barred Owls on long-term demography of Northern Spotted Owls. The Condor: Ornithological Applications 118, 57-116. DOI: 10.1650/CONDOR-15-24.1.

Header image: Spotted owl chicks via Tom Kogutus/USFWS.