More than 1,000 mallard ducks have died along a single creek in southern Idaho, and officials on Wednesday tested tissue samples to find out why.

The symptoms — lesions in the lungs and hemorrhaging in the heart wall — likely point to a bacterial infection, not avian flu, said Dave Parrish, regional supervisor for the Idaho Department of Fish and Game.

State wildlife biologists and U.S. Department of Homeland Security investigators were not ruling out any cause of death.

Migrating mallards from Canada and their local cousins were still perishing at Land Springs Creek near Oakley, about 180 miles southeast of Boise. Birds stagger and struggle to breathe before collapsing, said Parrish.

"There were dead mallards everywhere — in the water and on the banks. It was odd, they were in a very small area," Parrish said.

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

Tissue samples from the ducks and water samples from the creek were sent to the U.S. Fish and Wildlife Service national laboratory in Wisconsin, the University of Idaho and Washington State University. Results were expected Thursday.

The remote waterway is surrounded by farm land. A cattle feedlot is close by and several corn and alfalfa feeds ring the nearby town of Oakley.

Parrish said the ducks may have eaten grain treated with pesticides, or farming chemicals might have spilled into the creek.