Migration is the seasonal movement of birds, generally between breeding and non-breeding areas. The most common reason to migrate is the availability of food, shelter, and water at different times of the year. For instance, the lack of insects and plants in the winter means there is less food to eat.
To prepare for migration, birds become hyperphagic. That means they eat more food, which is stored as fat for their long journey. Fat is normally 3% to 5% of the birds mass. Some migrants almost double their body weights by storing fat before migration. The ruby-throated hummingbird weighs only 4.8 grams and can use stored fat to fuel a non-stop, 24-hour flight across a 600-mile stretch of open water from the U.S. Gulf coast to the Yucatan Peninsula of Mexico! And it’s even more amazing when you realize that most of the birds are only a few weeks old.
When you think about migration you may not realize that a majority of small song birds begin their journey at dusk and fly throughout the night. Scientists believe that birds fly at night because it’s easier to avoid predators and the calmer, cooler weather at night is better suited to their marathon trips.
Birds are also sensitive to weather conditions. There is no set date for migration. Birds can literally be blown in your yard for a pit stop and then ride out on the next good wind. Most night-migrating songbirds fly below 2000 feet when flying over land, but occasionally, fly higher to reach favorable winds.
So how do they know where to go? The upper beak of a bird has magnetite receptors that act like a GPS to determine which way they're flying. Along with this internal compass that uses the Earth’s geomagnetic field, birds use light, stars, and other external cues to guide them on remarkably long journeys.
Click HERE for the link to a special broadcast from NPR’s Science Friday which discusses bird migration, how birds orient themselves, what bird banding projects reveal and how scientists track birds during migration using Doppler radar and microphones.