Flying to a distant land, all for what?

Every spring, thousands of birds - some weighing a pound or less - set off on a journey of Herculean proportions.

They fly thousands of miles to breeding grounds in Saskatchewan and Alberta in Canada and to US destinations that include Alaska, Montana and North Dakota. Come September, they fly back over the same gigantic distances to autumn and winter habitats ranging from South Carolina to Florida, and from Texas to Mexico.

Why do they do this? Seasonal migrations of birds have long been one of nature's great mysteries.

For precise answers

Evolutionary theory offers some hypotheses, but scientists have long wanted to track down more precise answers.

The first step is to track the birds as they fly. And that is exactly what scientists are now doing for the first time.

In the Bear River National Wildlife Refuge on the northern shores of the Great Salt Lake in Utah, US, a marbled godwit, a shorebird is retrieved by Adrian Farmer, of the US Geological Survey, and Bridget Olson, of the US Fish and Wildlife Service.

Farmer and Olson attach a small box with a wire that weighs half an ounce to the back of the bird and set it free. A solar panel on the tiny backpack attached with the small box provides power to send a signal for six hours each day to satellites.

Rule of thumb: The maximum weight you can load on a bird is 3 per cent of its body weight.

Sassy's story

Using the satellite signals, the scientists tracked the bird. In a little less than a day, they found it flew 600 miles to Saskatchewan. In view of her destination, the scientists named this bird Sassy.

A second bird, caught a few days later, wound up in Alberta after making a pit stop outside Two Dot, Montana, US, where a lake and wetland offered temporary accommodation. The researchers named this bird Berta.

As to why the birds take the trouble to do all that flying, Farmer said it was too early for research to suggest an answer: "One theory is these birds evolved in the south, but over time they escaped predation in the tropics by going north to breed."

Kerry Hecker, manager of Canada's Last Mountain Lake National Wildlife Area, a migratory-bird sanctuary where Sassy ended up, said the northern wetlands also provide a rich source of food for the breeding birds.

Few months later, Berta was tracked returning to the US, very close to where she had been caught, near the Great Salt Lake. Eventually, she flew to southern California and the Baja peninsula, Mexico.

But here the story takes a sombre turn. As months passed, Sassy seemed stuck in Saskatchewan.

For several weeks now, the signals suggest she has not moved more than 500ft. Finally, the Canadian Wildlife Service was asked to take a look.

The lost bird

When Canadian wildlife officials went out to look for Sassy, the area they found themselves in had very tall grass and shrubs - inhospitable terrain for a shorebird that likes flat, open terrain. They realised they had been looking in the wrong spot. The correct location pointed by satellite signals was about 200ft away, said Ross Dickson, a wildlife technician at the Last Mountain Lake wildlife area.

More searches, however, including a dry-lake bed area from where a signal was received did not reveal the bird or transmitter. "There is always a chance that the bird could return to the same area [next year]," Dickson said. "We've got to live on hope here."