The migration of most birds is a yearly cycle. Not all birds migrate. The more severe the climate of an area, the greater percentage of nesting birds migrate. Two-thirds of bird species found in the United States migrate, some only short distances to more southern states. Those that do migrate have adaptations not seen in their non-migratory relatives.
Migratory birds can build fat stores as an energy source for long flights.
Migratory birds usually have longer, more pointed wings and weigh less than related non-migratory birds.