At least 2,000 mallard ducks have died within a few days in a remote area of southern Idaho, a development that confounded state wildlife officials Wednesday.
Only mallards are affected -- not other duck species or crows, eagles, magpies or geese.
"Quite honestly, we don't know what's going on," said Dave Parrish, regional supervisor for the Idaho Department of Fish and Game.
"It's a small, localized area -- we've never seen anything like this before."
Wildlife investigators have been collecting the birds along Land Springs Creek, near Oakley, about 150 miles southeast of Boise, since the weekend.
A hunter found 10 dead ducks along the creek Friday. When Fish and Game agents arrived on the scene Saturday, they discovered more than 500 dead birds, and the numbers keep increasing.
Parrish said that although authorities weren't ruling anything out, the symptoms the ducks displayed and necropsy results so far pointed to a bacterial infection.
Lesions have been found on the lungs of the birds and evidence of hemorrhaging in the heart walls. Authorities are especially puzzled that the malady seems confined to mallards. "You would normally think that a bacterial infection would affect all the waterfowl in the area," Parrish said.
But Mark Drew, a wildlife veterinarian with the department, thinks that may be explained by the small area involved. The dead ducks have been found along about a half-mile stretch of the creek.
Water and soil samples have been taken in the area, and scientists suspect that chemicals from nearby feedlots or farms may have contaminated the creek.
The U.S. Department of Homeland Security is also taking part in the investigation, mainly to establish whether the die-off has any link to avian flu.
Parrish calls that chance "highly remote."
Drew agreed. He said the Pacific Flyway Council, which is administered by the U.S. Fish and Wildlife Service, "has tested more than 20,000 birds across the Western United States -- there's no evidence that avian flu is anywhere in North America."
"The odds of it suddenly appearing in a small creek in southern Idaho are very small," he said.
Tissue samples from the birds have been sent to labs in three states. Results are expected as soon as today.