Researchers trapped 30 white-crowned sparrows as they made their annual migration from breeding grounds in Alaska to winter nesting sites in the southwestern United States and Mexico.

The birds were flown in the windowless pet compartments of commercial jets from the west coast city of Seattle, Washington to New Jersey, a state some 3,700 kilometres away on the east coast of the United States.

A few days later the birds were released with a tiny radio transmitter attached to a piece of cotton glued between their shoulders.

Their flight paths were followed by researchers on the ground and in a small plane and a very simple pattern emerged: the 15 adult birds realised they needed to fly southwest to get to their nesting grounds while the 15 juveniles, who had never made the trip before, simply continued to fly south.

Scientists have long been puzzled over how migratory birds are able to find nesting sites thousands of kilometres away even when they do not have other birds to guide them.

The white-crowned sparrow typically flies alone and at night as it makes its long journey and the ability of juveniles to find the winter nesting grounds on their own indicates an innate orientation.

Navigational map
The fact that the adults were able to correct their route after a 3,700-kilometre diversion suggests the birds may also build a navigational map as they migrate the thousands of kilometres to and from their winter nesting grounds, the authors concluded in the study published in the Proceedings of the National Academy of Sciences.

'On the basis of one migratory journey from Alaska to southwest North America, white-crowned sparrows obtain information that allows them to reach their wintering grounds from an area that their normal migratory route does not encompass,' wrote lead author Kasper Thorup of Princeton University's department of ecology and evolutionary biology.

'Our experiment indicates that the navigational map of adult white-crowned sparrows encompasses at least the continental United States and allows them to correct for vast displacements very rapidly - within days, at least - hinting that migratory birds may possess a global navigational map.'

While researchers have tracked birds using bands on their legs, including those shipped far away from their typical migratory routes, this is the first study in which displaced birds were tracked as they flew.

The displaced sparrows 'made rapid decisions about their migratory orientation after displacement,' the study said.

They took less than three days to fly more than five kilometres from the release site and reached the end of the observational zone of a 25-kilometre radius in an average of seven days. The birds were tracked for up to 122 kilometres after release. -- AFP