Each year, during the summer, thousands of spectacled eiders flock to Alaska's western and northern edges. They feed and breed on the river deltas, coastal plains, and marshy tundra. Then they vanish.
Exactly where the threatened sea ducks spend their winters has long been a mystery of the avian world. Now biologists have cracked the case.
In late winter, federal biologists Bill Larned and Greg Balogh found an estimated 140,000 spectacled eiders - perhaps almost the whole world's population - living in small holes in the ice of the frozen Bering Sea more than 100 miles (160 kilometers) off Alaska. It was the first time that large flocks had been seen during the winter.
From a distance the concentration of eiders looked like dark smudges on a white canvas, said Mr. Larned, who spotted them on an aerial survey. Closer up, he could see individual birds packed together to keep warm in minus 20-degree F. (minus 29 C.) temperatures.
Tipping off the biologists on where to look for their winter range was a single unexpected blip from a satellite transmitter attached to a female eider - the only one of two-dozen with working batteries.