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A recovery boat is seen Wednesday on the Mildred Lake tailings pond in northern Alberta. ((James Hees/CBC News))

Oilsands giant Syncrude says 350 birds have now died after landing on the Mildred Lake tailings pond Monday night, while Suncor said the grim tally on its ponds rose to 40 ducks.

On Tuesday, it was revealed that about 230 ducks had to be euthanized after landing on tailings ponds in the Fort McMurray area belonging to Syncrude. The province later announced ducks had also landed on tailings ponds belonging to Suncor and Shell.

Tailings ponds are a toxic slurry of chemicals and hydrocarbons used to separate bitumen from sand.

Reporters were invited to visit Syncrude's Mildred Lake facility Wednesday where a full-scale recovery operation is underway. Several boats are trolling for birds while others on shore are using nets. It's expected the operation will take another week.

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Syncrude CEO Scott Sullivan speaks to reporters at the Mildred Lake tailings pond Wednesday. ((CBC))

Syncrude CEO Scott Sullivan said the number of dead waterfowl will continue to climb. Speaking at the recovery command post, he said he's disappointed the deaths occurred despite the company's deterrent system.

He said sound cannons and radar were working and deployed at the time the ducks landed.

"Staff were able to get out in the boats right away and get the birds off the water right away," he said.

Syncrude will perform a thorough investigation, with weather as the main focus, Sullivan said. He expects the investigation will take months.

Rain hindered birds' flight: Syncrude

The companies are citing freezing rain in the area as a major factor in the "unusual bird activity." The rain made it difficult for the birds to fly. Birds that landed on roads and parking lots were easily approached, likely meaning they were fatigued.

"With [a] fully deployed deterrent system, we have not had this experience," said Sullivan. "This is new for us. That's why it is critical for us to do a thorough investigation to really understand if there are learnings and forward action." 

Sullivan says the company plans to bring in independent scientists to help find out what happened.

A University of Alberta biologist planning to study deterrent systems said freezing rain could have played a role.

"The fact that they weren't moving away when they were approached is a very important piece of information that really does suggest that they were at their physiological limits," said Colleen Cassady St. Clair. "They could not fly any longer."

Environmentalists said weather shouldn't matter.

"Alberta's weather changes all the time," said Mike Hudema with Greenpeace. "If tar sands companies can't ensure the safety of birds, animals and humans regardless of the weather, they should be shut down until they can."  

Federal Industry Minister Jim Prentice and Alberta Environment Minister Rob Renner said charges may be laid.

"Certainly, it's awful, and we need to get to the bottom of it to find out what happened and if necessary, charges will be laid once more," said Prentice.

The latest incidents came days after Syncrude agreed to pay a $3-million penalty for the deaths of 1,600 ducks in another tailings pond in April 2008 in a deal its lawyers reached with provincial and federal Crown prosecutors.