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migration of birds (very short note)

Bird migration is the regular seasonal movement, often north and south along a flyway between breeding and wintering grounds, undertaken by many species of birds. Migration, which carries high costs in predation and mortality, including from hunting by humans, is driven primarily by availability of food. Migration occurs mainly in the Northern Hemisphere where birds are funnelled on to specific routes by natural barriers such as the Mediterranean Sea.Birds migrate to move from areas of low or decreasing resources to areas of high or increasing resources. The two primary resources being sought are food and nesting locations.

Birds that nest in the northern hemisphere tend to migrate northward in the spring to take advantage of burgeoning insect populations, budding plants and an abundance of nesting locations. As winter approaches, and the availability of insects and other food resources drops, the birds move south again.

Escaping the cold is a motivating factor but many species, including hummingbirds, can withstand freezing temperatures as long as an adequate supply of food is available.
Types of migration

The term migration is used to describe movements of populations of birds (or other animals). One way to look at migration is to consider the distances traveled.

    Short-distance migrants: May move only a short distance, as from higher to lower elevations on a mountainside.
    Medium-distance migrants: Some species may cover distances that span from one to several states.
    Long-distance migrants: Birds that typically have ranges that extend from the United States and Canada in the summer to Mexico and further south in the winter.


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