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Understanding Migration

Why Some Birds Make Seasonal Journeys


Understanding Migration
Photo © Duckycards / iStockphoto.

Migration, like flight, is a phenomenon not limited to birds. In general, migration is the annual movement of animals between their breeding grounds and wintering sites. Migration occurs in response to changing seasons and is predictable and repeated each year.

Migration is not the only type of animal movement. Some animals travel long distances foraging for food (see Scientists Follow Frigatebird's 26-Day Journey for an example of this). Some animals move to new locations as soon as they are old enough to venture beyond the location where they were born. Such movement is called dispersal. Animals may also move out of an area at unpredictable times due to harsh conditions or limited resources (this is called irruptions or invasions). The key to migration is that it is predictable, seasonal, and repeated each year (Sibley 2001, 59-60).

There are two types of migrating species that can be distiguished. Complete migrants are species whose entire populations migrate between breeding and wintering grounds. Partial migrants are species in which some individuals do not migrate and instead spend the entire year in either the breeding or wintering ground (while other individuals migrate).

In general, migrating birds follow flyways or pathways between breeding and wintering grounds. It is common though for birds to disperse during their migration and stray long distances from normal routes.


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