SALEM, Ore. -- A pesticide is suspected in the mysterious deaths of dozens of federally protected geese this month near Keizer.

State and federal wildlife investigators hope to find out by this week exactly what killed about 60 geese at Staats Lake between April 10 and 14, according to Dale Mitchell, assistant administrator of the pesticide division at the Oregon Department of Agriculture.

In the meantime, Mitchell said investigators are surveying farms in the area for potential off-label use of zinc phosphide, a pesticide widely used to control voles in grass seed fields.

But he said he would be surprised if it was the cause of the deaths because the agriculture department has been notifying farmers recently about the consequences of misusing zinc phosphide.

Brad Bales, a wildlife biologist for the Oregon Department of Fish and Wildlife, said the geese also could have died from aspergillosis, a respiratory disease contracted from eating moldy grain and a frequent cause of geese deaths in the Willamette Valley.

The agriculture department sent samples of the dead birds this week to its laboratory in Portland. Geese carcasses also were sent to the U.S. Fish and Wildlife Service's National Wildlife Health Center in Madison, Wis.

Toxicology results from the federal lab aren't expected until the end of the month.

If pesticide poisoning is found to be the cause, Mitchell said it could lead to further restrictions of above-ground use of zinc phosphide in grass seed.

"Certainly misuses could bring about additional restrictions," Mitchell said.

Growers are restricted from above-ground application of zinc phosphide in grass seed between Sept. 1 and April 30 to protect migratory geese.

Zinc phosphide is a relatively harmless pesticide to most animals but is highly toxic to geese.

The geese found dead at Staats Lake have been identified as Canada cacklers, which nest in Western Alaska and spend winters in Oregon.

All geese, whether resident or migratory, are protected under the federal Migratory Bird Treaty Act.