Many people know that bird migration has been going on for ages (millions of years, in fact), and that it happens twice a year. But when you stop and ponder the details, migratory birds' epic journeys are nothing less than astonishing.

Consider the improbability of a migratory bird, which may weigh a few ounces or less, safely finding its nesting site after flying thousands of miles, braving storms and evading predators — only to reverse the journey a few months later. Now, multiply those odds by several billion — the number of birds migrating — and it's hard to deny that we're once again experiencing one of nature's grandest miracles.

How do birds successfully migrate? Over time, they've mastered the mechanics of long-distance flight, developing a superb sense of direction and seemingly endless endurance. Supplied with these exceptional abilities, and gifted with light-weight frames and powerful wings, migratory birds perform eye-popping feats on a regular basis. We've collected five facts highlighting some of the fantastic achievements of these intrepid long-distance travelers.

Bar-headed Geese flying at high elevation. Photo by Wang LiQiang/Shutterstock.

1. Airline passengers aren't the world's only high-altitude travelers: Bar-headed Geese have been observed migrating at elevations topping 23,000 feet in the Himalayas. Roaming the frigid skies around Mount Everest is no small feat, and for most animals — including humans — these heights are basically off-limits. Even well-equipped mountaineers with oxygen masks and polar expedition gear can have trouble functioning at this altitude. Not so for the amazing geese that brave these oxygen-starved elevations armed with nothing but their own stamina and wing power.

Northern Parula, back on territory. Photo by Ray Hennessy/Shutterstock.

2. Many migratory birds, but not all, fly at night, navigating in the dark. That's right, when you're in bed napping, their wings are flapping. Why? Colder air temperatures reduce the danger of overheating and allow birds to fly further without having to stop and cool down. Equally important, dark skies tend to hold fewer predators.

For birds, migrating at night is more risky than it used to be. Most notably, the steady-burning lights atop communication towers attract and disorient birds. Television, radio, and cell towers cause up to 7 million bird collisions each year in North America. Learn how you can help alleviate this threat.

Hummingbird in morning sun. Photo by Ramona Edwards/Shutterstock.

3) Migratory birds can remember and return to the exact location where they were born. We know that they navigate using the sun, stars, and Earth's magnetic field. But exactly how they do this remains a mystery, partly because migrating birds use a combination of senses, including, in at least one species, smell.

Compared to the uncanny ability of migratory birds, the human navigational system — the one we're born with — isn't always up to the task. To give one famous example, when Mark Twain found he was expected to learn 1,200 miles of the Mississippi River before becoming a riverboat captain, he expressed an immediate desire to die. Yet for migratory birds, a journey of this length is not unusual, or even extremely long — some birds fly almost six times that far without stopping.

Scientists can now accurately track these travels thanks to the use of tiny geolocator backpacks that transmit birds' locations. This information sheds new light on migration, while helping to pinpoint locations in need of conservation.

Snowy Egret in flight. Photo by Relentless Images/Shutterstock.

4) Migratory birds travel at the same speeds we usually do while driving. These range from 15 to 55 miles per hour, depending on the species, prevailing winds, and air temperature. At these rates, migratory birds typically fly from 15 to 600 miles — or more — each day. Birds at the upper end of this range are covering some serious ground: To keep up with them, you'd need to run the entire width of the state of Montana in a day.

The danger of moving over long distances, or even short ones, has grown for birds as human-built structures and the glass windows they hold have proliferated. Because windows are usually invisible to birds, they cause fatal collisions with frightening regularity. In fact, window collisions account for up to 1 billion bird deaths in the United States each year.

Are your windows bird-safe? If you're not sure, visit our windows solutions page to find out how you can help prevent collisions.

Arctic Terns over the Arctic Ocean. Photo by Tony Brindley/Shutterstock.

5) The Arctic Tern can fly more than 49,700 miles each year, racking up more miles than any other migratory bird. Traveling between Arctic breeding sites and Antarctic summering grounds takes Arctic Terns on an annual journey that is roughly equivalent to flying twice around the globe. But the title for longest nonstop flight goes to the Bar-tailed Godwit, which covers nearly 7,000 miles without a rest.

Once they land, migratory birds need to be on the lookout, and not just for their natural predators. Free-roaming cats threaten birds worldwide. They were introduced into the United States several hundred years ago, and are now the country's number-one direct, human-caused threat to birds, killing approximately 2.4 billion wild birds there each year. Learn how you can help combat this danger and keep cats safely restrained.