Zt: Although some facets of waterfowl migration remain puzzling, research has fused certain pieces of the puzzle. Because we are currently experiencing or anxiously awaiting fall migration, I’ll summarize some of our knowledge on why, when, and where waterfowl migrate during fall in North America.

Why do waterfowl migrate? According to Dr. Bellrose, “the biological reason for migration is survival.” Additionally, survival is a prerequisite for reproduction. These two life-history components are all that matter to waterfowl and all wildlife. Hence, through eons and natural selection, waterfowl have evolved behavior to avoid intolerable conditions (e.g., cold and ice, food inaccessibility, disturbance, predation, etc.). They move from one location to find suitable habitats and resources elsewhere, ultimately to survive and produce offspring. Thus, most species of North American waterfowl endure the physical perils and energetic costs of migration each fall by leaving their breeding grounds and flying hundreds to thousands of miles to wintering grounds to survive and subsequently reproduce in spring.
http://www.ducks.org/DU_Magazine/DUMagazineSeptOct2003/1791/Conservation.html