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The Pacific Flyway is one of the four major migration route for waterfowl in the United States, Canada and Mexico. In the early 30's, waterfowl biologists used band returns and other information dating back to the early 1900's to help identify primary waterfowl migration routes, such as the Pacific Flyway, which link breeding grounds in the north to more southerly wintering areas. The four primary North American flyways that have been named are the Atlantic Flyway, the Mississippi Flyway, the Central Flyway and the Pacific Flyway. In the U.S., the Pacific Flyway includes Alaska, Arizona, California, Idaho, Nevada, Oregon, Utah, Washington, and those portions of Colorado, Montana, New Mexico, and Wyoming west of the Continental Divide.
Except along the coasts, the flyway boundaries are not always sharply defined and both in the northern breeding, and the southern wintering, grounds there is more or less overlapping. As a matter of fact, in the region of Panama, parts of all four flyways merge into one.
The routes followed by migratory birds are numerous, and while some of them are simple and easily traced, others are extremely complicated. Differences in distance traveled, in time of starting, in speed of flight, in geographical position, in the latitude of the breeding and wintering grounds and in other factors all contribute to great diversity. No two species follow exactly the same path from beginning to end; geographical groups of species with an almost continental distribution may travel different routes.
Bird migration is generally thought of as a north-and-south movement, with the lanes of heavier concentration following the coasts, mountain ranges and principal river valleys. In general, it may be said that the great routes of migration do conform very closely to major topographical features when these happen to lie in the general direction of the travel to be performed. It happens to work out nicely in North America where the coasts, mountain chains and come of the larger rivers do not depart from a north-and-south alignment.
The terms "migration route" and "flyway" are to some extent theoretical concepts, while the latter has, in addition, come to have an administrative meaning. Migration routes may be defined as the lanes of individual travel from any particular breeding ground to the winter quarters of the birds that use them. Flyways, on the other hand, may well be conceived as those broader areas in which related migration routes are associated or blended in a definite geographic region. They are wide arterial highways to which the routes are tributary.
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