Spokeswoman Jolanta Kowalski says lab results have determined a bird found on Christian Island two weeks ago died of Type E botulism.
Kowalski says that's the only test that has come back, but other samples of the thousands of dead birds have been sent for testing.
She says test results from gulls picked up at Wasaga Beach on Oct. 6 are expected next week.
Kowalski says ministry staff were out earlier this week picking up between 1,000 and 2,000 dead birds that washed up on the shoreline in the Wasaga Beach area.
Botulism had been the suspected cause and Kowalski says such die-offs are common in the fall and residents shouldn't be concerned if dead birds wash ashore.
"It could continue for a few more weeks depending on water temperature, waves," Kowalski said Thursday.
Previous die-offs were caused by the waterfowl eating botulism-laced fish but the large number this fall is unusual.
"We strongly suspected (botulism) at the time, but this is the first confirmation of Type E botulism," Kowalski said.
Kowalski said residents are being warned to keep a tight rein on their pets.
"We recommend that pet owners don't let their dogs run loose and eat a carcass," she said, adding it's not a threat to human health.
"You can't get it by drinking the water and if you do have any fish that you might be concerned about, the toxin is easily destroyed by heat," Kowalski said.
Bottom-feeding fish ingest toxins that cause botulism and the birds feast on the dead or dying fish. Fish react to the toxin by becoming erratic, making them an easier prey target for a loon or duck that's looking for something to eat.
About a decade ago some 25,000 birds died on Lake Erie from eating botulism-laced fish.