Bird migration is the temporary movement of birds from one place to another, and the primary motivation for migration appears to be food. Birds migrate to move from areas of low or decreasing resources to places where resources are high or on the rise. If all birds were to stay in the same rich, tropical areas year-round, food would become scarce and breeding would be less successful with undernourished parents and hungry hatchlings. But as food sources regenerate in northern regions each spring, millions of birds migrate to those areas to take advantage of the abundance.

While short-distance migration probably developed from a fairly simple need for food, the origins of long-distant migration patterns are much more complex. They’ve evolved over thousands of years and are controlled at least partially by the genetic makeup of the birds. They also incorporate responses to weather, geography, food sources, day length, and other factors. The mechanisms initiating migratory behavior vary and are not always completely understood. Migration can be triggered by a combination of changes in day length, lower temperatures, changes in food supplies, and genetic predisposition.

In addition, the longer days of the northern summer provide extended time for breeding birds to feed their young. This helps diurnal birds to produce larger clutches than related non-migratory species that remain in the tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.