Wow, just wow. But it's easy to dismiss that as a trick of perspective until it was documented: It's also fascinating that they evolved to fly at night when it is colder because the air is denser. And they hug the land contour which intuitively seemed inefficient, going up over and over instead of staying up. And they have incredible endurance: I'm always amazed at birds who with tiny bodies and not a lot of fat reserves can tolerate such cold temperatures. It's incredibly cold at 24,000 feet.