anisodactyl

Anisodactyl refers to a common toe arrangement in the feet of many perching birds, especially passerines, in which three toes point forward and the hallux, or rear toe, points backward. The term comes from Greek anisos, meaning unequal, and daktylos, meaning finger.

In anisodactyl feet, toes II, III, and IV project forward to grasp branches, while the hallux reverses to secure the perch from behind. This configuration provides a versatile and stable grip for perching, climbing, and foraging on uneven substrates. The arrangement supports rapid adjustment and strong grasping power, aiding birds that spend much of their time on twigs and foliage.

Anisodactyly is the most widespread foot pattern among birds, particularly among passerines. It contrasts with other arrangements such as zygodactyly, where two toes face forward and two backward (found in parrots and woodpeckers), and pamprodactyly, in which all toes may be rotated forward, as in some swifts. Some species exhibit variations or specialized adjustments, but the basic anisodactyl pattern remains dominant in a large majority of birds that perch.

While primarily associated with perching and arboreal foraging, anisodactyl feet also contribute to maneuverability in dense vegetation and diverse substrates. The trait is typically discussed alongside other foot arrangements in studies of avian locomotion and evolution.