Dead bird carcasses are lining South Cole Road. Concerned drivers are asking why.

The word that one state wildlife biologist uses is “disturbing.”

He’s referring to the possibly couple of hundred bird carcasses that line the gravel along both sides of South Cole Road, near CS Beef Packers south of Boise.

The birds are ring-billed gulls and California gulls, according to Bill Bosworth, a wildlife biologist with Idaho Fish & Game.

Thousands of gulls nest in a settling pond on private property southwest of the beef packing plant. The large number of bird deaths in recent years has concerned passersby and led some to wonder if they’re being poisoned by something in that pond or others nearby.

Bosworth said Fish & Game has been investigating, and they have determined two causes for the bird deaths.

“The main cause is the gulls in these ponds, the juvenile birds, can’t fly well,” Bosworth said. “They tend to come out on the road, and there’s a lot of industrial traffic. They get hit by cars.”

The fledglings begin leaving the nests in early to mid-June, and some of the late developers are learning to fly into July.

Last year, Fish & Game also had some of the dead birds tested for toxins and pathogens. Some were found to have died from salomonellosis, a disease caused by the bacteria salmonella.

All birds are susceptible to salmonella infection, according to the National Wildlife Health Center. Their survivability depends on several factors, including bacterial virulence.

Bosworth said the bird deaths were happening at least three years ago and possibly longer — before the beef packing plant was built.

Bosworth believes one of the reasons the birds are using the settling pond along Cole Road is because it’s fenced, possibly protecting the nesting birds from predators.

An employee at a nearby business said the pond is on the property of a company that’s based in Nampa; officials with that company could not be immediately reached Tuesday.

Fish & Game officials plans to talk with the property owner about ways to reduce the bird deaths. One possibility is putting up a fence that could keep some of the fledgling birds off Cole Road. Another idea is hazing the birds so they don’t nest there.

Katy Moeller: 208-377-6413, @KatyMoeller