Question of the week:
Why don't birds fall out the tree whenthey sleep?
A. Birds have a unique arrangement of tendons that automatically tighten when the bird is asleep. As the bird settles into a resting position, the large flexor tendons in the toes are stretched in such a way that the toes are automatically pulled inward, increasing the strength of the grip. This allows the bird to grab a few restful winks without the fear of falling out of the tree.
To see this in action, stop by your butcher shop and see if you can obtain a recently removed chicken leg with the foot still attached. A little gross, maybe, but you should be able to examine the heel area for a couple of threadlike tendons.
Pull on the tendons and you will see the toes curl inward. This is the same action exhibited by a perching bird. As the bird moves into a resting state, often with its head resting on his back, the tendons in its legs and feet tighten, insuring the tight grip required to remain upright and in the tree, even while asleep.