For anything that flaps and flies, the most important part of the wingbeat cycle is the downstroke. Pulled by the big pectoralis muscle, the wing, when fully extended, crashes through the air to create the thrust that keeps a bird airborne. But you also have to get the wing up in order to get it back down again, and fast enough so that you don’t lose what you gained in the downstroke, says Samuel Poore, a graduate student in evolutionary morphology at Brown University. Poore knows upstrokes: he and his adviser Ted Goslow have dissected the wing mechanics of European starlings--and have shaken up some beliefs about the evolution of flight.
During an upstroke, a bird rapidly folds its wings into its body, bending at both elbow and wrist, and then zips its wings back up to an elevated position, above and behind the back, to begin the next downstroke. But in small, fast-flapping birds like the European starling, researchers have had a difficult time seeing exactly what the wing bones, muscles, and tendons do. Poore and Goslow got around that by clamping the birds and electrically stimulating their muscles when their wings were placed at various positions of the wingbeat cycle.
One muscle used in the upstroke is the supracoracoideus, which attaches to the keeled sternum on the bird’s chest at one end and the humerus (the upper arm bone) at the other. Previously, researchers had thought that this muscle simply helped elevate the humerus, pulling it straight up. But Poore and Goslow found that when the muscle contracts, it rotates the humerus around.
The supracoracoideus does its spinning act because of the unique way it attaches to the humerus. Although the muscle originates at a point on the back of the sternum below the shoulder, a tendon attached to the muscle slips through a canal (dotted line in drawing at left) in the shoulder and wraps partly around the top of the shaft of the humerus, like a string around a top. When the muscle shortens, it elevates and spins the humerus into the correct position for the next downstroke, says Poore.
The find sheds new light on the evolution of flight. Early fossil birds, including the Jurassic-period Archaeopteryx, don’t have a shoulder canal for the supracoracoideus (hints of it aren’t seen for another 10 million years); instead the muscle passed over the shoulder at such an angle that it couldn’t effectively rotate the humerus. We can’t say that Archaeopteryx couldn’t fly, Poore says. But because it didn’t have this organization, the supracoracoideus wasn’t able to rapidly rotate the humerus into a perfect downstroke position. That probably limited its ability to carry out slow flight in takeoff and landing, which is when you need really rapid wingbeats.