Anatomy and histochemistry of spread-wing posture inbirds. 4. Eagles soar with fast, not slow muscle fibres.

Slow fibers are typically characterized as functioning in avian postural behaviors such as soaring flight, and are described for a number of elite soarers such as vultures, pelicans and albatrosses. Golden Eagles and Bald Eagles also display soaring behavior and we examined their flight muscles for the presence of slow fibers. Surprisingly, eagles lack a deep layer to the pectoralis found in other soaring species. Additionally, the pectoralis as well as other shoulder muscles had few to no slow muscle fibers. The lack of functionally meaningful numbers of slow muscle fibers in eagle flight muscles indicates that they must rely on fast fibers for posture; these can function in that role due to their high aerobic capacity and also perhaps a "tuning" of muscle contraction frequency to function more efficiently at isometric contractions.