migration in birds
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.
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.
The pattern of migration can vary within each category, but is most variable in short and medium distance migrants.
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat, or weather. Sometimes, journeys are not termed "true migration" because they are irregular (nomadism, invasions, irruptions) or in only one direction (dispersal, movement of young away from natal area). Migration is marked by its annual seasonality.
In contrast, birds that are non-migratory are said to be resident or sedentary. Approximately 1800 of the world's 10,000 bird species are long-distance migrants.