Researchers say they have learned new details that explain why invasive barred owls are thriving while native spotted owl populations are slowly disappearing.
David Wiens, a biologist with the U.S. Geological Survey, conducted a 3-year study on competition between the species in partnership with Oregon State University.
The barred owl and the spotted owl are like a pair of siblings. They are closely related species that compete over everything, from nesting sites in old growth snags to juicy flying squirrels, a prime owl food source.
Wiens radio-collared and tracked barred and spotted owls at a research site in the Oregon Coast Range. Barred owls clearly have the edge in the completion. Wiens found 82 pairs of them at his site, compared to just 15 pairs of spotted owls.
Diet appears to be one key reason barred owls are thriving. Spotted owls exclusively eat a few nocturnal rodents: flying squirrels, wood rats, and mice. Barred owls also prey on those species, but they have a much broader diet. Wiens examined barred owl pellets, and found they eat everything from beetles to snails to fish. His team observed one female owl spending hours perched in a big leaf maple tree above a deep pool.
“Finally we went over and looked where this bird was perching, and we found the remains of just hundreds of crayfish there,” Wiens says.
Wiens says barred owls appear well adapted to aquatic and stream environments.
“We had another case where a barred owl was wading around in a stream, eating very small aquatic snails,” he said. “We’d find pellets from this bird that had 60, 70 of these very small snails in them.”
Wiens says that broad diet helps a greater density of barred owls occupy smaller, more fragmented patches of old-growth forest. His study also found that barred owls fledged, on average, six times more young than spotted owls, which don’t produce young every year.
This density of competitors appears to be pushing spotted owls out of the best nesting sites, and placing pressure on their already limited food source, nocturnal rodents.
The U.S. Fish and Wildlife Service is planning a series of experiments that will kill and remove some barred owls in study areas in the northwest. But federal biologists have suggested that given the density of barred owls living in the Northwest, it may be difficult to kill enough of them to make a difference.
Complicating matters further, it’s not entirely clear if barred owls should be considered an invasive species, or a species that naturally expanded its range over time.
“We’ll never know if this range expansion is natural or human-caused, and science just isn’t going to tell us,” says Eric Forsman, a Forest Service researcher who has studied spotted owls for decades.
Forsman and other researchers stress that protecting old growth habitat is the best way to reduce the intensity of the competition between the two species, and to give the spotted owl a chance to survive.
“If you start shooting barred owls, you’re going to have to keep shooting them for a long time, because as soon as you stop, they will move back in,” Forsman says.
Forsman says he thinks the two species may be able to coexist, but with fewer spotted owls than in the past.