The anatomy and histochemistry of flight hindlimb posture in birds. II. The flexed hindlimb posture of perching birds.

During flight, birds employ one of two hindlimb postures. Perching birds utilize a flexed posture with their folded legs tucked beneath the body, whereas shorebirds and raptors use an extended posture with straightened legs trailing behind the body. Maintenance of either posture during flight requires the hindlimbs to hold their position for prolonged periods. Slow contracting fibers are known for their fatigue-resistant properties and are often found in high percentages in muscles utilized for postural actions. Given the similar actions required of the hip and knee flexors used during flight, we hypothesized that the equivalent postural muscles of perching birds (flexed posture) would contain similar percentages of slow fibers as shorebirds (extended posture). We investigated the anatomy and fiber type composition of seven hindlimb muscles in yellow-headed and red-winged blackbirds and revealed that they possess a smaller percentage of slow fibers than we found previously in the same muscles of American avocets and black-necked stilts. The comparably smaller body size of yellow-headed and red-winged blackbirds could mitigate the need for more slow fibers. In addition, the biomechanical placement of the weight force in the flexed posture may require less muscle force for postural support during flight and, therefore, fewer slow fibers.