Dead geese found in Hayden and on the Rathdrum prairie this year died from ingesting rodent pesticides, according to the Idaho Department of Fish and Game.

Kara Campbell, regional wildlife biologist, said the department received calls in autumn about dead Canada geese on the prairie, and from a resident in Hayden who noticed a Canada goose that appeared sick.

“The goose was reported to have a large bulge at the base of its neck, was acting lethargic and would not fly,” Campbell wrote on a Fish and Game blog. “My co-worker and I went out to examine the goose and determined that indeed something was not right.”

Biologists investigated further and learned about 20 geese had been reported as being sick or dead.

“At this point, we decided if we received more phone calls about dead geese, we would ask for them to be brought into the IDFG office,” Campbell wrote.

The department sent four geese to the USGS National Wildlife Health Center in Madison, Wis., for a necropsy.

Biologists learned the geese tested negative for lead poisoning and avian influenza, and asked the lab tests be continued.

“A few more weeks went by with no word, but then I received an email from the USGS lab with results indicating phosphine poisoning as the cause of death,” Campbell said.

The phosphine likely resulted from ingestion of zinc phosphide, a bait common to agricultural areas where it is used to kill rodents such as mice, gophers and ground squirrels.

Biologists aren’t sure how the geese ingested the poison.

“I still don’t know how the geese got into the zinc phosphide, but this brings up an important message about how crucial it is to properly apply all rodenticides and pesticides near your home or business,” Campbell wrote. “Proper application of these substances is extremely important to protect people, pets and wildlife.”

Zinc phosphide is a relatively short-lived toxin, according to IDFG. The department has not received any more calls of sick or dead geese.