Snow geese, which spend their summers on Arctic shorelines, spend their winters on the western side of the Gulf of Mexico, which has not been affected by the BP oil spill. (CBC)Arctic birds that migrate south for the winter could be affected by the massive Gulf of Mexico oil spill if it continues to spread, the World Wildlife Fund warns.

While they are spending the summer in Canada's North, tens of thousands of snow geese and other birds will fly to their winter nesting grounds in the southern United States and Mexico later this year.

Many birds stop in marshes and wetlands along the Gulf of Mexico coast to rest and refuel, and that has the conservation group worried about what will happen to those birds if BP's spill keeps growing or if the oil is not cleaned up by then.

"The real danger is that they could get covered in oil … which will lead to them losing the insulating effect of their feathers, and they could then die of exposure," Craig Stewart, Arctic director with World Wildlife Fund Canada, told CBC News.

"Secondly, their food source will potentially not be there."

The WWF predicts upwards of 200,000 birds of all kinds will die as a result of the spill, which began when an oil rig exploded on April 20.

Winter grounds not yet affected

But Stewart said how many migratory birds could be harmed or killed, or become too sick to fly north next year, is hard to estimate.

In the case of snow geese, Stewart said they migrate to the western side of the Gulf of Mexico, where the slick has not yet spread.

"We expect that the birds that you would find over in the central Arctic, in the Kivalliq region [of Nunavut], would be probably from that area. And if the oil spreads then it might be an issue," he said.

"When they return down there in the fall, if it's not cleaned up in time, it could have a bit of an effect.

Inuit hunters worried

Inuit who hunt snow geese from the spring well into the fall say they are worried about the oil spill's possible impact on the birds. (CBC)"Those that do frequent the coast of the gulf … if they can't track down food because of the oil, then you would have problems with them making it all the way back up to the Arctic afterwards," he said.

Stewart's concern is shared by Inuit who rely on snow geese as a food source.

"This time of the year, in the springtime — and even the fall time, when they're migrating south — we still hunt them," said Johnny Mamgark, an Inuit hunter from Arviat, Nunavut.

Mamgark said local elders have expressed fear that the Gulf of Mexico spill could harm the southbound snow geese.

"They've seen the birds being affected down there, but they talk about the birds that do migrate up here. So yeah, people are worried up here," he said.

Whooping cranes not at risk

Meanwhile, one of the most fragile northern bird species, the whooping crane, will likely not end up in the path of the Gulf of Mexico spill, said Brian Johns, a retired biologist with the Canadian Wildlife Service.

About 263 whooping cranes — the largest known wild flock of the species — are currently nesting in Wood Buffalo National Park, which straddles the Alberta-Northwest Territories border.

In the winter, the flock travels south to the Aransas National Wildlife Reserve in southern Texas, where they spend their winters.

"The birds leave Wood Buffalo National Park and move down to Saskatchewan. And then from there, their migration is pretty well direct down to the south Texas coast," Johns said.

As a rule, the migrating whooping cranes do not stop anywhere along the part of the Gulf coast where the oil spill is spreading, Johns said.

"It's due west, about 800 kilometres of where the slick is originating," he said, adding that the ocean current is taking the oil spill away from the Texas wildlife reserve.

The reserve has set up protective barriers along nearby waterways, just in case, Johns added.