Reasons for Migration
Birds that migrate are those that spend parts of the year in different areas according to what food is available and seasonal weather change. Some species only migrate a short distance in response to abrupt weather changes, but some migrate thousands of miles. Many birds, including warblers and shorebirds, make long migrations in the spring and fall. In the spring, the birds travel to their breeding areas. In the fall, they travel to their wintering grounds. These places can be thousands of miles apart.
Most migratory bird species travel along a "flyway." The long-distance fliers tend to travel in concentrated areas. In the Northern Hemisphere, this is generally south for the winter season and north in spring to breed and raise young. These flyways are paths through the sky that are traveled by many species. Across the United States, there are four major flyways where birds congregate on their northern and southern journeys: the Atlantic, Mississippi, Central and Pacific. Some birds might migrate a shorter distance or undertake vertical migrations if they live in a mountainous location.
No one is quite sure how bird navigation works, but the long-distance fliers usually travel at night at high elevations. Nocturnal migrants may be avoiding predators, or simply being forced to fly non-stop over inhospitable terrain or water. The bright lights of large urban areas can disorient the birds and cause them to fly in the wrong direction or even into the light source.
Many larger bird species such as geese fly along their migration routes in flocks to help reduce individual stress and save energy by flying in formations that decrease wind resistance, such as the V-formation.
According to the Forest Service, more than 300 of the 850 bird species in North America spend the summer in the United States, and winter in Mexico and South America. Many species utilize coastlines to travel, whether for the habitat it provides---many species of waterfowl and shorebirds, for example, are migratory---or the strong, reliable seashore breezes, especially helpful for soaring birds like raptors. Some birds even go out to sea after the young have been reared. Most notable in this category are puffins, which travel from their nesting colonies to the open water, where they form large "rafts" of birds. During heavy travel periods---at the peak of spring or fall migration---a given habitat may be flooded with non-resident birds, adding exoticism and excitement to the local ecosystem.
Some birds such as the red-breasted nuthatch are considered irruptive migrants, meaning the number that migrate and the winter range varies greatly each year. In this case, the Eastern year-round range is expanding southward.