The Pacific Flyway is a major north-south flyway for migratory birds in America, extending from Alaska to Patagonia. Every year, migratory birds travel some or all of this distance both in spring and in fall, following food sources, heading to breeding grounds, or travelling to overwintering sites.
Any given bird species travels roughly the same route every year, at almost the same time. Ornithologists and "bird lovers" can often predict to the day when a particular species will show up in their area.
Waterfowl arriving in California's Central Valley in the fall
Along the Pacific Flyway, there are many key rest stops where birds of many species gather, sometimes in the millions, to feed and regain their strength before continuing. Some species may remain in these rest stops for the entire season, but most stay a few days before moving on.