You probably see birds flying by on a daily basis, but have you ever thought about how they do it? Clearly people in earlier generations did. Before the secrets of heavier than air flight were sorted out, our ancestors fooled around with a number of bird wing contraptions. These all failed because the mechanism of bird flight is actually much more complicated than anyone knew back then. The YouTube series Smarter Every Day explains why that is.The first thing to know is that birds are not flying by simply pushing air downward. The mechanism at work is actually much more akin to a fixed-wing aircraft, but the way a flapping wing gets there is very different. All flying birds have long flight feathers along the edge of the wing. On each down stroke, the feathers overlap so they all push together. On the backstroke, the feathers spread out, letting air pass through them to reduce resistance.This two action system ensures that the higher pressure stays where it’s supposed to — on the underside of the wing. This gives the bird upward lift not unlike an airplane, but it happens through physical movement. Bird wings are essentially acting like a one-way check valve on each and every stroke.Slow motion video shows just how efficient bird flight can be. The low pressure system on top of the wing is strong enough to lift up smaller feathers mid-flight. This is a highly adaptive system that can allow birds to alter wing shape to glide and maneuver in a variety of ways. Some birds can even hover in a way planes can’t. It wouldn’t be practical to try to replicate this system for non-biological flight.