Roadside barn owl mortalities on I-84 concern researchers

BOISE – A speckled wing pokes out from the bunch of grass beside Interstate 84, wobbling in the wind that scours the scrubby high desert.

Graduate student Than Boves pulls his truck to the side of the road and picks up the lifeless bird with his bare hands, examining its dusty brown body before putting the bird in a plastic bag.

The carcass is a barn owl that likely met its end as it tracked a rodent to the pavement, reacting too late to an oncoming vehicle. It’s a scene repeating itself at an alarming rate on the interstate, raising environmental, safety and agricultural concerns.

Many of these dead owls are finding their way into a freezer bursting with feather-filled plastic bags at the Raptor Research Center off Capitol Boulevard in Boise. This is where Boves, who is studying raptor biology at Boise State University, is conducting an exhaustive, low-tech study on barn owl deaths between Boise and Burley.

Up to 2,500 owls have been killed along the roadway in the past two years, and researchers are worried the deaths may be greatly reducing the population, even raising the spectre of localized extinctions.

Barn owl numbers have been declining around the world for decades, but the numbers in southern Idaho are particularly striking because no one thought many lived in the area to begin with, Boves said. Southern Idaho is the northern edge of the barn owls’ inland North American range, which stretches from coast to coast. In Europe, some countries with road mortality rates similar to south Idaho have seen localized extinctions.

No one knows how many of the owls live in southern Idaho. The birds are nocturnal and secretive, making population studies difficult, said raptor biologist Carl Marti, who has done extensive studies on the owls. But Marti said the numbers from the study likely mean local populations are being heavily affected.

“I’ve been rather surprised at the numbers he’s been finding out there,” he said.

Boves is working with BSU professor Jim Belthoff on the study and has been doing his twice-monthly surveys for two years. During that time he’s found more than 800 dead barn owls. Accounting for owls he doesn’t find and carcasses carried away by scavengers, he estimates as many as 3.5 times that many have been killed.

On a recent survey, Boves found about 15 owls from the edge of Boise to Burley. And that’s just a piece of Interstate 84. There’s anectodal evidence that the road mortality extends to other south Idaho roads and farther down the freeway.

It’s fairly clear the owls are being hit by vehicles. But why barn owls are dying at a rate more than 10 times that of the next-most-frequently found bird is unknown, Boves said. The owls appear to be hunting before they die. Some have been found with a vole in their talons and many have undigested voles in their stomachs, and likely follow rodents to the road and sometimes across it.

The numbers have surprised everyone, including the researchers conducting the study, Boves said.

One theory is that the owls are getting nonlethal poisoning from pesticides in their agricultural hunting areas, Boves said. The poisoning could affect the central nervous system, making them a little slow to react to vehicles.

Boves has been conducting surveys for two years but has yet to see a barn owl get hit. In fact, he estimates he’s seen only three living barn owls in the wild.

For every nesting owl killed, scores of rodents get a reprieve. A barn owl nest can have six to 11 young, each capable of eating a rodent a day, and the owls often nest twice a year, said Bill Mattox, president of the Conservation Research Foundation. That means fewer rodents chewing crops and is the reason Mattox’s environmental research foundation is trying to convince farmers to install nest boxes for barn owls in southwest Idaho.

Though the I-84 study involves an area composed of 40 percent agricultural land, 60 percent of the owls have been found in agricultural areas, meaning they likely ate rodents that fed on crops.

Owls can save farmers thousands of dollars a year in pesticides, one reason farmers should be concerned about the deaths, Mattox said.

“Most farmers recognize the benefit of these birds but, nevertheless, today there’s a remnant of the old mentality of, ‘If it’s a hawk or an owl, you shoot it,’ ” Mattox said.

The precise impact barn owls have on rodent populations is not known, but their decline could be a bellwether for other as-yet-unknown issues as problems lower in the food chain make their way up to predators, Belthoff said. In the 1960s raptors began disappearing as females were laying eggs with thin shells, a problem eventually linked to the pesticide DDT. The chemical was banned in 1972.

“Often predators are really good indicators for environmental problems,” he said.

Belthoff and Boves are still studying why owls are more likely than other birds to be killed by cars. Eventually, they hope to find ways to prevent the deaths.

One experiment Beltholff said that drivers are more likely to see: Road signs with an owl likeness and a warning to watch out for the birds while driving at night.