While looking at the little screech owl, I took a series of photographs and made this gif to illustrate the of the automatic grasping action of the talons.

The structure of bird feet is set up so that the foot automatically grasps when the ankle joint is bent.  

This automatic grip allow birds to sleep while perching, and for raptors clench/grasp prey as the leg is folded on impact.

The mechanism of the foot is ingenious…. there’s no muscle in there at all. 

The foot is powered entirely by a pulley system of tendons. 

Two tendons that run along the back of the leg, Flexor Digitorum Longus and Flexor Hallucis Longus are responsible for the automatic grasp. The former pulls the forward facing toes, and the latter pulls on the hallux, or back toe. 

I drew a schematic diagram of these two tendons here:

It’s particularly interesting in raptors.

Raptors swoop down on prey with talons/legs outstretched. The impact with the prey folds the raptor’s legs against its body, causing the talons to clench automatically, tearing into the prey. The automatic grip is strong enough to kill, and is what allows many hawk species to catch and kill other birds in midair. 

The ingenuity and perfection of this mechanism is mind-blowing.