As autumn approaches the tundra days begin to shorten, the temperature begins to fall and the abundant food supply begins to disappear. The waterfowl gather in large flocks and head for the more accommodating swamps, marshes and flooded croplands of the south. While the trigger mechanism for migration may remain the same, the logistics become more and more difficult to ascertain when biologists begin looking at the grand scheme. When we speak of the mechanics of migration we have to speak in the broadest sense because there are so many variables. There are also those species, like the peregrine falcon, that appear to be erratic post-nesting wanderers that don't have a particular wintering ground yet return to the same nesting site year after year.
Animals appear to rely on a combination of senses and stimuli during migration.
Sight, smell, celestial navigation, perhaps memorization and even the earth's magnetic field, all seem to play a part. How these combinations are used, when and to what extent, are scientific speculation. Take the magnetic field, for example. Monarch butterflies routinely migrate over 2,000 miles from Maine to Mexico, to overwinter in the same grove of trees that their ancestors have overwintered in for hundreds of years. The creature that returns to Mexico in the fall is usually four generations removed from the butterfly that left Mexico in the spring.