Brown and Harvard scientists have learned that a single ligament at the shoulder joint stabilizes the wings of birds during flight.From the press release:
To better understand how the birds stabilized their wings during flight, they used CAT scans to make a 3D “virtual pigeon skeleton” and calculated the forces needed to maintain a gliding posture. They found that neither the shoulder socket nor the muscles could keep pigeon wings stable.
The critical player, they found, is the acrocoracohumeral ligament, a short band of tissue that connects the humerus to the shoulder joint. The ligament balances all of the forces exerted on the shoulder joint – from the pull of the massive pectoralis muscle in the bird’s breast to the push of wind under its wings – making it a linchpin for modern bird flight.
To find out if this ligament played the same shoulder-stabilizing role in primitive animals, the team looked to the alligator. In the laboratory the scientists put three alligators on motorized treadmills and took X-ray video. Baier and Stephen Gatesy found that alligators use muscles – not ligaments – to do the hard work of supporting the shoulder.
In examining the fossil record (i.e. Archaeopteryx, Confuciusornis, Sinornithoides, & Sinornithosaurus) they found that the new ligament-based force balance system appears to have evolved more gradually within Mesozoic fliers.
“What this means is that there were refinements over time in the flight apparatus of birds,” Baier said. “Our work also suggests that when early birds flew, they balanced their shoulders differently than birds do today. And so they could have flown differently."