Definition: Each year, several billion birds migrate, and at any one point in time bird migration is occurring somewhere in the world. Migration is a seasonal event, in which certain species travel between breeding grounds and overwintering grounds in order to take advantage of seasonal changes in resource availability. Globally, around 40% of bird species migrate; however, the proportion of species that are migratory in any particular location increases with distance from the tropics. Since most of the global land mass is in the Northern hemisphere, the most common migratory pattern is a northward spring migration to breeding grounds, where young are raised, followed by a southward autumn migration in order to escape harsh northern winters. However, a wide range of alternative migratory patterns are common. Although a simplification, key migration routes, or flyways, can be identified, with some species migrating up to 35,000 miles annually. Birds rarely complete long-distance migration without several stops.
Trends: Bird populations and their migration patterns are to some extent dynamic, and changes in the timing and range of bird migrations have been observed over the past 20 years. There is some evidence that particular birds have shifted their ranges to higher latitudes, or altitudes; however, it is not possible to generalize about the impact of climate changes on bird distribution and movements since the effects are very species specific. Depending on the reproductive cycle and migration pattern of the species, both delays and advances in the timing of spring migrations have been observed in Europe in response to warmer springs.
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