Annually many bird species migrate as their food supply disappears, water sources freeze, shelter options lessen, and daylight hours reduce. Migration cannot begin, however, until the young birds have fledged in their breeding range and all birds have eaten extra food for the long journey. Many species double their weight to fuel their migration. Purple Martins, as an example, eat three times their weight in mosquitoes in a day in preparation for the trip.
Generally, birds migrate when the weather and atmospheric structure are good. This means winds are blowing in the direction that they are headed and tail winds are available to increase their speeds. Navigational options for birds include use of the stars, sun, and internal sensitivity to the Earth's magnetic field as their compass(es). They also learn routes from other birds. Often they will use one of the four major flyways in the USA: Atlantic, Central, Mississippi, or Pacific. For information on flyways click here and here.
Birds using powered flight that involves continuous flapping and a level course through the air are often nocturnal migrants. At night the air is calmer or more stable and predators are fewer. These can be viewed with a telescope as they pass by the light of a full moon. Soaring birds, on the other hand, prefer daylight when they can ride thermals of rising hot air.
Blackbirds migrating south.
He published his recommendations in the Fall 2009 issue of the Audubon magazine.
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