The avian hindlimb, commonly known as the leg, is a highly specialized structure adapted for a variety of functions beyond simple walking. It is developed for bipedal movement, which is walking on two limbs, and is instrumental for perching, swimming, catching prey, and absorbing the shock of landing after flight. This limb’s unique architecture, characterized by significant bone fusion and reduction, allows for a framework that is both lightweight and rigid. The specialized leg provides the necessary leverage and strength for supporting the bird’s body weight.
Connection to the Body and Thigh
The hindlimb connects to the body through the pelvic girdle, which is extensively fused with the vertebral column to create a single, rigid structure called the synsacrum. This synsacrum is formed by the fusion of numerous vertebrae, often ranging from 10 to 23, including the thoracic, lumbar, sacral, and some caudal vertebrae, creating a strong assembly for limb attachment. The innominate bones of the pelvis—the ilium, ischium, and pubis—also fuse to the synsacrum, forming a broad, fixed support system unlike that of mammals. This extensive fusion provides structural support and rigidity necessary to withstand the forces of flight and terrestrial movement.
The synsacrum’s rigidity is a necessary adaptation that helps stabilize the body during the powerful movements of the legs and wings. The first bone of the hindlimb, the femur, articulates with the hip socket (acetabulum) formed by the innominate bones. It is generally a stout, short bone that functions more as an extension of the hip than as a primary element for stride length. In flying birds, the femur is often held in a horizontal orientation, which positions the knee joint closer to the body’s center of gravity. This positioning brings the limbs forward beneath the body, improving balance and efficiency during walking.
The Tibiotarsus and Fibula
The segment directly below the knee joint is composed of two bones: the tibiotarsus and the fibula. The tibiotarsus is the principal bone of the lower leg, resulting from the fusion of the tibia with the proximal tarsal bones. This fusion creates the longest and largest bone in the avian pelvic limb, providing a strong, single-unit anchor for muscle attachment. The resulting bone is a major weight-bearing element, and its length can vary considerably depending on the bird’s habits, such as being much longer in wading birds.
The fusion of the tibia and tarsal bones enhances the structural integrity of the lower limb. The fibula, the second bone in this segment, runs parallel to the tibiotarsus but is significantly reduced in size. In most species, the fibula fails to reach the ankle joint, typically extending only about two-thirds of the way down the leg. This reduction in the fibula streamlines the limb’s mass while maintaining the strength provided by the robust tibiotarsus.
The Specialized Avian Foot
The distal segment of the hindlimb, often mistaken for the shin or the lower leg, is the tarsometatarsus. This bone is formed by the fusion of the distal tarsal bones with three metatarsal bones, specifically those corresponding to digits II, III, and IV. The tarsometatarsus acts as a third segment of the leg, lending extra height and leverage for activities such as running, jumping, and landing. Its shape and length are highly variable, with species like long-legged waders having a tarsometatarsus nearly equal in length to the tibiotarsus.
At the end of the limb are the digits, or toes, which are composed of individual bones called phalanges. Most birds possess four digits, with the number of phalanges in each digit typically following a formula such as two, three, four, and five, starting from the innermost toe. The arrangement of these digits is highly adapted to the bird’s lifestyle. The most common pattern is the anisodactyl foot, where three toes face forward and the first toe (hallux) points backward, providing an efficient grip for grasping branches.
Another common configuration is the zygodactyl foot, which features two toes forward and two toes backward, giving the foot an X-shape. Birds such as woodpeckers and parrots exhibit this arrangement, which provides a stronger, more secure grip for climbing tree trunks or holding food. The terminal phalanx of each digit is tipped with a horny sheath that forms the claw.