Migration is a seasonal movement from one area to another - usually a breeding and a nonbreeding area. Migration evolved in response to environments that change with the seasons. Migration allows birds to take advantage of productive seasons and avoid unproductive seasons. Most migrant birds spend only two to four months of the year on their nesting grounds. The majority of the year is spent elsewhere.

Not all birds migrate. Some find the resources they need throughout the year, others switch to different food sources as the seasons change, and a few become inactive during lean times.

Prior to migration, birds build up body fat to burn as fuel during their journeys. Many birds have traditional "refueling' stops along their migration route where they fatten up for the rest of the trip. Some land birds, such as warblers and hummingbirds, take long over-water flights where running out of gas could be fatal!

Birds use a variety of cues to navigate, including the sun, stars, the Earth's magnetic fields, visual landmarks such as mountains, rivers, or coastlines, and even odors! The exact ways in which birds use these cues is still largely a mystery.

Not all migration involves long flights. Many birds which nest at high elevations simply move down slope to lower regions for the winter months.