Thousands of dead birds that washed ashore along a stretch of Georgian Bay were to be picked up starting Monday, said a spokesman for Ontario's Ministry of Natural Resources.

"We will have a crew going out to clean up the provincial park beaches in this area," said John Cooper, a ministry spokesman based in Peterborough, Ont.

Initial estimates pegged the number of dead waterfowl as high as 6,000, but Cooper said ministry staff were going to do an updated assessment on Monday.

Tests were being done to determine the cause of the deaths, but previous such die-offs often were caused by the waterfowl eating botulism-laced fish, Cooper said.

"It's not uncommon to have these kind of die-offs of fish and birds on the Great Lakes at this time of year, but in this location and this number is unusual."

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, Cooper said.

Cleanup to focus on provincial park

The die-offs along a roughly three kilometre stretch on Georgian Bay is the largest Ontario officials have seen in many years. About a decade ago some 25,000 birds died on Lake Erie from eating botulism-laced fish.

The government cleanup will centre on a large provincial park on the shores of southeast Georgian Bay, but private landowners in the area near Wasaga Beach will be left to their own devices, Cooper said.

"They can leave the carcasses there and let nature take its course, they can bury the carcasses on the property or they bag them up and put them out with ... the garbage," he said Sunday in a phone interview.

It was unclear if more birds would wash ashore, but Cooper said it was possible the problem would persist for several weeks.

Type E botulism toxin is produced by a bacterium that lives in lake bottom sediment, and under certain conditions it begins producing the toxin, which then enters the aquatic food chain, according to the ministry. Birds who eat affected fish can die.

Botulism toxins are easily destroyed by heat, meaning fish and birds that people catch pose no risk provided they're properly cooked, Cooper said.

However residents in the area are being warned to keep a tight rein on their pets and prevent them from getting near the dead birds and fish on the beaches.