Bird feet
Anisodactyly. This is the most common arrangement of toes in birds, with three toes forward and one back. It is the basic pattern in passerine birds, especially the songbirds, and hunting birds like eagles, hawks, and falcons. The anisodactyl arrangement lets passerine birds perch on vertical surfaces, such as trees and cliffs. The hind toe joins the leg at the same level as the front toes. The leg arrangement of passerine birds has a special adaption for perching: a tendon-locking mechanism. A tendon in the rear of the leg is automatically pulled and tightened when the leg bends. This makes the foot curl stiffly round when the when the bird lands on a branch. The tendon is locked in place by another tendon. So passerines sleep while perching without falling off. Certain species of passerines have stiff tail feathers, which help the birds balance themselves when perching on vertical surfaces. Syndactyly is like anisodactyly, except that the third and fourth toes  or three toes, are fused together. This is characteristic of Coraciiformes (kingfishers, bee-eaters, rollers, and relatives).