Passerine (Wikipedia)
Anatomy; Origin and evolution sectionsQuote, Anatomy; Origin and evolution sections
A tendon in the rear of the leg running from the underside of the toes to the muscle behind the tibiotarsus will automatically be pulled and tighten when the leg bends, causing the foot to curl and become stiff when the bird lands on a branch. This enables passerines to sleep while perching without falling off.
Nature Atlas Long-Form Articles, First Edition