At least 4,000 species of bird are known to be regular migrants, which is about 40 percent of the total number of birds in the world.
(Although this number will likely increase as we learn more about the habits of birds in tropical regions.)
Birds can reach great heights as they migrate.
Bar-headed Geese are the highest-flying migratory birds, regularly reaching altitudes of up to five and a half miles above sea level while flying over the Himalayas in India.
But the bird with the record for the highest altitude ever is the Ruppel’s Griffon Vulture.
The Arctic Tern has the longest migration of any bird in the world. They can fly more than 49,700 miles in a year, making a round trip between their breeding grounds in the Arctic and the Antarctic, where they spend their winters.
Over a lifespan of more than 30 years, the flights can add up to the equivalent of three trips to the moon and back.
The Northern Wheatear travels up to 9,000 miles each way between the Arctic and Africa, giving it one of the largest ranges of any songbird.
What makes this an especially amazing feat is that the tiny bird weighs less than an ounce.
The Bar-tailed Godwit has the longest recorded non-stop flight, flying for nearly 7,000 miles, over eight days, without food or rest.
To prepare for the extremely taxing effort of migration, birds enter a state called hyperphagia, where they bulk up on food in the preceding weeks to store fat, which they’ll later use for energy on their long journeys.
Some, like the Blackpoll Warbler, almost double their body weight before flying 2,300 miles for 86 hours without stopping.
Even birds that don’t fly migrate.
Emus, the large Australian birds, often travel for miles on foot to find food, and many populations of Penguins migrate by swimming.
Migration can be terribly dangerous for birds, and they often don’t make it back to their starting point.
Sometimes natural occurrences like harsh weather play a role, but human activities are the cause of many deaths.
In the United States alone, up to one billion birds die each year from window collisions,
seven million from striking TV and radio towers.