Migration of birds is something studied by thousands of scientists, and which is not yet fully understood. In general, birds migrate to live where the weather best suits them. Some birds migrate thousands of miles each year between their nesting grounds and their summer and fall feeding grounds. Up to 80% of birds in North America migrate.
Birds migrate at different times of the year and even different times of day, all based on their own internal clocks and rhythms. Why birds migrate is still being debated - some cases are obvious, but others are completely mysterious. Some birds migrate when it gets too cold for them to stay where they are, but others migrate from one location of plenty to another, for seemingly no reason.
The longest known migration route is that of the Arctic Term, who move over 11,000 miles each way to travel bewteen their preferred locations near the north and south pole.