Anisodactyly

Anisodactyly is a term used in avian anatomy to describe a common arrangement of digits on the feet of many perching birds. In anisodactyl feet, the hallux (digit I) is directed backward and opposes the three forward-facing digits II, III, and IV, creating a configuration with three toes pointing forward and one pointing backward.

This toe arrangement is the most widespread among passerines (perching birds) and is well suited to grasping branches and perching on uneven surfaces. The functional advantage lies in a stable grip and versatile manipulation of substrates, aiding perching, locomotion, and foraging.

There is variation among species in toe length and joint mobility, but the basic anisodactyl pattern—one backward-facing toe and three forward-facing toes—remains a defining feature in many birds. The term is primarily used in ornithology to describe foot morphology in perching birds, though related toe arrangements are described in other bird groups.

Related concepts include zygodactyly, where two toes face forward and two backward (found in parrots and woodpeckers), and pamprodactyly, where all toes can face forward and rotate toward the bill. Etymology derives from Greek anisos (unequal) and daktylos (finger or toe), reflecting the uneven arrangement of digits.