When & How Do Birds Know to Migrate?

When & How Do Birds Know to Migrate?

WHEN AND HOW DO BIRDS KNOW TO MIGRATE?    

External factors - Change in the amount of daylight, availability of food and change in the weather.           Internal factors - Genetic aspect.  Example would be the Orchard Oriole, they are complete migrators that leave long before the weather changes or food becomes scarce.

Migratory Restlessness
As spring or fall approaches, small migratory birds tend to become restless and get more active after sunset.
Flight by day or by night
Large birds such as Hawks, Cranes, and Geese migrate during the day. They rely on rising columns of warm air, called thermals.  Birds ascent to the top of the column and are able to glide effortlessly.  Smaller birds such as Orioles, Hummingbirds, and Warblers migrate almost exclusively at night.  Flying at night allows the birds to rest and feed during the day.  The atmosphere at night is much more favorable for night flight.  Also, there are fewer predators at night.

Navigation Maps
Birds have some kind of internal navigation map.  Birds are sensitive to the pull of the earth's magnetic poles.  Also, many studies show birds migrate via their genetic coding or by learning the route from their parents.  It is amassing  how precise they are year after year.