Great Horned Owl
The great horned owl’s toes and talons lock round limbs and hold the bird firmly to sleep.
In the great horned owl, the extensor (fused) tendons of the leg, which retract the toes and clinch the claws, pass through a groove in the tarso-metatarsus and then into and through a hole in the bone, as a rope passes through a pulley-block. The tendons are thus held firmly in place as they pass over and through what in the human being would be the heel. When the leg is bent and drawn up, the tendons are also drawn tight, clenching the toes and driving in the talons.
Thus, when the owl settles down on a limb, its toes and talons lock round it and hold the bird firmly in sleep.