Bird migration refers to the regular seasonal journeys undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. These however are usually irregular or in only one direction and are termed variously as nomadism, invasions, dispersal or irruptions.

Migration is marked by its annual seasonality. In contrast, birds that are non-migratory are said to be resident or sedentary. Many bird populations migrate long distances along a flyway.

The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer and returning in the fall to wintering grounds in warmer regions to the south. The primary advantage of migration is conservation of energy. The longer days of the northern summer provide greater opportunities for breeding birds to feed their young.

The extended daylight hours allow diurnal birds to produce larger clutches than those of related non-migratory species that remain in the tropics year round. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.

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