Flight published an article by a Dr E H Hankin who gives an account of his study of birds while in flight.
He observes that birds do not have a muscle that can bend the wing tip downwards by direct action. But he promises to explore this matter in another chapter. “In bats”, he says, “there is such a muscle, which can bend the wing downwards at the carpal joint, and is used in flapping flight at the end of each downstroke.
“In birds, any appearance of bending downwards at the carpal joint can only be due to indirect causes, such as pressure of air on the upper surface of the wing. Slight rotation round the axis of the wing can occur at the carpal and metacarpal joint, and is so produced by the muscles that I am about to describe…”
And watch out for the amusingly childlike bird drawings on the following page.