BOISE, Idaho - The number of mallard ducks that have died along a creek in southeastern Idaho has climbed to 2,500, as puzzled wildlife officials awaited test results they hoped would provide clues to what is killing them.

Idaho Department of Fish and Game and U.S. Department ofHomeland Security officials expected to have results Thursday from the tests on tissue samples from the ducks' abdominal tract and on water samples from the creek.

The battery of tests at the U.S. Fish and Wildlife Service's national laboratory in Wisconsin, the University of Idaho and Washington State University were expected to rule out an avian flu outbreak.

The Idaho Department of Fish and Game and the U.S. Department of Homeland Security were testing tissue samples Wednesday, hoping to rule out an avian flu outbreak.

The ducks mysteriously began dying last week around Land Springs Creek, near the remote town of Oakley, about 180 miles southeast of Boise.

Some migratory mallards from Canada and their local cousins were still perishing at the creek Wednesday, staggering and struggling to breathe before collapsing, said Dave Parrish, regional supervisor for Fish and Game.

"I've never seen anything like this in 20 years here," he said. "There were dead mallards everywhere - in the water and on the banks. It was odd, they were in a very small area."

The massive outbreak is vexing scientists because only mallard ducks are dying. Golden eagles, geese, magpies, crows and other birds in the area all remain healthy.

Tissue from the ducks' intestinal tract and water samples from the creek were sent to the Fish and Wildlife Service national laboratory in Wisconsin, the University of Idaho and Washington State University. The agencies expect to review test results Thursday to determine the cause of death.

Mark Drew, a wildlife veterinarian with the stateDepartment of Agriculture, said the ducks likely were exposed to a single contamination source and gathered at the creek, their mutual roosting point, to die. He did not suspect the mallards were passing a contagious virus.

The ducks may have contracted a bacterial or fungal infection by eating grain treated with pesticides by local cattle farmers, Drew said. Farming chemicals may also have spilled into the small spring-fed creek, which measures just 3- to 6-inches deep.

In addition to Idaho Fish and Game and Homeland Security officers, representatives from the U.S. Department of Agriculture, the Idaho Department of Environmental Quality and the local health district were investigating the deaths.

The agencies posted signs warning hunters not to eat any birds killed near the creek.

"I'd say there's no reason for alarm in the sense that literally the sky is falling and there's disease spreading," Drew said. "It's unusual in the number of birds and the sense that it's only mallards, but it's nothing that would cause anyone to panic."