A team led by David Baier, a post-doctoral research fellow in the Department of Ecology and Evolutionary Biology at Brown University, focused on the shoulder joint since it's a place where powerful forces exerted by muscles converge.

The researchers used computerized tomography imaging to create a 3D "virtual skeleton" and then calculated the forces needed to maintain a gliding posture. They found neither the shoulder socket nor 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, making it a linchpin for modern bird flight.

"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," Baier said. "Some scientists think they glided down from trees or flapped off the ground. Our approach of looking at this force balance system can help us test these theories."

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