Only three populations exist, one in the Upper Midwest, a second in the Rocky Mountain region and a third that nests in Alaska and spends the winter along the Pacific coast.

• 目前世界上只存在着三个种群：一个在美国中西部以北，第二个在洛矶山地区，第三个群居于阿拉斯加，在太平洋沿岸过冬。