Figuring Out Birds' Migrations, Motivations
Monday, September 11, 2006
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."