King Eiders molt their primaries in August and September, and head feathers from late October through January. During the time of head feather molt most birds are in their wintering region. The three main winter regions in the Bering Sea differ in their carbon and nitrogen isotope ratios. King Eiders that grow feathers in those three regions incorporate the different isotope ratios into their feathers, allowing us to infer in which region they grew the feather based on the isotope ratios measured in feathers that we collect from birds on breeding grounds.

By using birds equipped with satellite transmitters we were able to delineate the isotopic ranges of the wintering regions in the Bering Sea. The range of feather isotope ratios from birds wintering in these regions are shown in the graph next to the map, with single feathers shown in the same color as the region where the bird wintered in.