Have you ever wondered why birds hang onto trees when they're asleep? Why don't they fall off?

The answer lies in the the hallux toe, which is the toe on some birds that points backwards instead of forwards. We already mentioned the hallux toe once on the show, when talking about how well-adapted some bird feet are for perching on branches.

But it also holds the bird in place even when it's sleeping. It's because of the way the tendons work in a perching bird's leg. When the bird hunkers down for sleep, it bends its leg at the knee and the heel. Two thin wires, called flexor tendons, run down the leg around the knee and heel, and attach to the toes.

The bending of the bird's legs when it perches tightens up these tendons, and, like a puppet's strings, curls the toes. The neat thing is the bird doesn't even have to think about it to make it happen.