Despite extensive recovery efforts, northern spotted owls in the northwestern U.S. are declining. Fast. According to a new Condor study, their populations have dropped by as much as 77 percent in parts of their range in just the last few years. What’s surprising is their biggest threat: not climate change or habitat loss, but other owls.
The U.S. Fish and Wildlife Service listed the northern spotted owl, Strix occidentalis caurina, as threatened in 1990 because their old-growth forest habitats were being destroyed throughout their range by logging and timber harvesting. To help with northern spotted owl management and conservation, a team led by Katie Dugger of the U.S. Geological Survey studied mark–recapture, reproduction, and territory occupancy data collected between 1985 to 2013 at 11 study areas in Washington, Oregon, and California. These sites represent 9 percent of the spotted owl range.
The team focused on survival, fecundity, recruitment, rate of population change, and local extinction and colonization rates, as well as how these factors were affected by habitat and climate changes. They also looked specifically at competition with the barred owl (Strix varia), a larger raptor from the eastern U.S. that’s been making its way westward. They’ve now invaded the entire range of the northern spotted owl.
The researchers estimate a range-wide decline of 3.8 percent a year over the study period of nearly three decades. Since monitoring began in 2009, spotted owl populations declined as much as 77 percent in Washington, up to 68 percent in Oregon, and up to 55 percent in California. Recruitment rates, or the amount of young owls that become part of the breeding population, were highest during drier, milder winters.
However, while climate and habitat changes impacted the owls, their biggest threat were the barred owls that competed with them for space, food, and habitat wherever they overlapped. The owl invaders were linked to decreased survival and increased local extinction rates of native spotted owls in all 11 study areas.
In fact, barred owl densities may now be high enough across the spotted owl's range that – despite continued management and conservation of suitable owl habitat on federal lands – the persistence of native owls are at risk in the long term. Although, removing barred owls might reverse (or at least slow) these declines: Their removal from the Green Diamond Resources study area had rapid, positive effects on spotted owl survival.
Barred owl with crow. Robert L Kothenbeutel/Shutterstock