No avian spectacle is grander or easier to appreciate than the fall and spring migrations along the West's two major north-south flyways—the Pacific Flyway and the Central Flyway. Mapped out, each resembles a river in the air, with tributaries flowing into and out of either end.
The statistics on migration are the stuff of the Guinness Book of Records: the Arctic tern may fly more than 11,000 miles from its breeding grounds in the Arctic to wintering areas in the Antarctic. Swallows and swifts travel nonstop, feeding in flight during their entire journey. And some songbirds lose half of their body weight during this avian endurance contest.
But some details of bird migration remain wrapped in mystery—scientists still do not fully understand how birds know when to depart on their journeys (day length appears to be one key trigger). Nor do they know exactly how birds navigate, returning to the same breeding and wintering areas year after year, despite changes caused by climate and construction.
The fall migration of waterfowl approaches its peak this month and next at many wildlife refuges in the West. (We list the best of them Fantastic Five: Top Western Birding Destinations.) Viewing the winter congregation of tundra swans at a refuge in the Klamath Basin of California and Oregon or on Oregon's Sauvie Island is enough to make you fall in love with birds.