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. ...
