In the long term, global warming is likely to let forests grow further north in the Arctic, squeezing the tundra breeding grounds of shorebirds - like curlews, sandpipers or red knots - into a narrowing belt bounded by the Arctic Ocean.

The report says the Arctic is heating twice as fast as the rest of the globe and that the warming "will have implications for biodiversity around the world because migratory species depend on breeding and feeding grounds in the Arctic."

Several hundred million birds migrate every year thousands of km (miles) to the Arctic to breed, largely because the chill region is almost free of egg-eating predators. They spend the Arctic winter in places from Patagonia to Mauritania.

"The bar-tailed godwit can fly from Alaska to New Zealand with no stops," Meltofte said. Populations already rise and fall according to the availability of food in the build-up to the giant flight, lasting almost a week.

"The birds don't want to go to the Arctic but they have to go to breed," Meltofte said. "Imagine if two million shorebirds stayed and laid eggs on the ground in Mauritania. The chicks would be eaten by the crows, gulls or jackals."

Any decline in bird populations due to disruptions in the Arctic will affect ecosystems in the south.

In the shorter term, however, a warmer Arctic may help some bird species by making food, such as insects for chicks, more available. One survey showed 12 percent of Arctic shorebird species were growing in numbers, 42 were stable and the rest falling.

The gray plover is among those gaining while the spoon-billed sandpiper of eastern Siberia is endangered.

Birds may be able to fly so far because few parasites live in the chill Arctic or in the salty tidal flats they favor during winter. Higher temperatures might spread parasites.

"Birds have this amazing ability to build muscle without training. All mammals need hard training to build muscles, birds do it naturally, all by hormones," Meltofte said.

The Arctic report blames emissions of heat-trapping gases from burning fossil fuels for rising temperatures and says the Arctic is warming quickly because dark water and ground, once uncovered, absorb more heat than snow or ice.