Every spring, thousands of birds, some weighing a pound or less, set off on a journey of herculean proportions. They fly thousands of miles north, to breeding grounds in Saskatchewan and Alberta in Canada and to U.S. destinations that include Alaska, Montana and North Dakota. Come September, they fly back over the same gigantic distances to fall 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. (Think about it: If you lived near a balmy beach in someplace nice and tropical, would you fly three thousand miles north just to lay your eggs?)
Evolutionary theory offers some hypotheses, but scientists have long wanted to track down more precise answers. There is growing urgency for this, too, because many species of shorebirds appear to be in steep decline, and it would be useful to find out whether the problem is in their breeding areas to the north, in their fall and winter habitats to the south, or on the journey between. Wherever the trouble is, it will be important to know where particular birds came from, because birds from different winter habitats may end up in the same breeding area, and vice versa.
The first step is to track the birds as they fly. And that is exactly what scientists are now doing for the first time.
When our drama opens, it is April in the Bear River National Wildlife Refuge on the northern shores of the Great Salt Lake in Utah. A marbled godwit, a shorebird, flies into what looks like a curtain of mist. Only it is something else, a human contraption -- a net. The bird is retrieved by Adrian H. Farmer of the U.S. Geological Survey and Bridget Olson of the U.S. 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 provides power to send a signal for six hours each day to satellites. Such devices have long been used on other animals, but the recent development of super-lightweight transmitters have for the first time allowed scientists to track birds that weigh as little as a pound. (The same technique is also being used to track birds that could be carriers of infectious diseases such as bird flu.)
Rule of thumb: The maximum weight you can load on a bird is 3 percent of its body weight.
Using the satellite signals and Google Earth's online global-imaging service, 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, Mont., where a lake and wetland offered temporary accommodation. (The researchers named this bird Berta, but, uncertain whether it was male or female, they left open the option of abbreviating it as Bert.)
The scientists focused on marbled godwits because the species has disappeared from some parts of its breeding range, especially in the eastern Dakotas and Minnesota, probably as a result of agricultural encroachment. Based on naturalists' reports from a century ago, the species' numbers also appear to be sharply down along the south Atlantic coast of the United States, especially in Florida.
As to why the birds take the trouble to do all that flying, Farmer said, it is 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."
Such journeys may have been relatively short when Arctic glaciers covered much of North America, but the birds may have gradually lengthened their journey as the polar ice caps shrank. 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.
The fact that Berta stopped at protected wetlands along the way shows the importance of these sites in maintaining such species, said Charles Duncan of the Western Hemisphere Shorebird Reserve Network, which works to conserve shorebird habitats. "I could hardly believe my eyes when I saw the map of these birds' travels," said Duncan in a statement, noting that the birds' odysseys have included three of the network's 64 protected sanctuaries all over the hemisphere. "It was like Sassy was on our payroll."
In late July, Berta was tracked returning to the United States, very close to where she had been caught in April, near the Great Salt Lake. Eventually, she flew on to southern California and the Baja peninsula.
But here the story takes a somber turn. As July gave way to August and then to September, Sassy seemed stuck in Saskatchewan. Daily signals downloaded from the satellite showed the marbled godwit hanging out in Canada long after she should have headed south.
In the last several weeks, the signals suggest she has not moved more than 500 feet. The transmitters have an accuracy of about 300 feet. Finally, Farmer and Olson asked the Canadian Wildlife Service in Saskatchewan to take a look.
"Wherever the transmitter is, it is open to the sunlight," said Farmer, as he raised the possibility that the backpack had fallen off. That, of course, is the optimistic scenario. The other possibility is that Sassy is dead or injured. "We are concerned about our baby."
When Canadian wildlife officials went out to look for Sassy last Wednesday, the area they found themselves in had very tall grass and shrubs, inhospitable terrain for a shorebird that likes flat, open terrain. But on Thursday, they realized they had been looking in the wrong spot -- the correct location, pinpointed by multiple satellite signals -- was about 200 feet away, said Ross Dickson, a wildlife technician at the Last Mountain Lake wildlife area.
Dickson said his hopes rose that Sassy had escaped her harness and is safe when he found no feathers in an area about the size of a football field -- that would have been circumstantial evidence that she had been killed by a predator. He was also reassured because he saw no fox or coyote tracks and, when he stuck his arm into an American badger hole, he found nothing.
More searches over the weekend, however, including in a dry lake bed area from where a signal was received on Sept. 3 did not reveal bird or transmitter.
"There is always a chance that the bird could return to the same area in 2007," said Dickson. "It does have a colored leg band on it, a blue leg band with white lettering. We've got to live on hope here."Washington Post
