First let's look at the range of motion in a human shoulder socket.

Extend your arms straight out in front of you, shoulder width apart.

Now lean forward at your waist so that your back is horizontal like a bird’s.

Now try to put your arms out straight, directly behind you while leaning forward.

Do not press your arms up or back any further than you comfortably can!

You have probably found this motion difficult (if not painful).

But a bird’s shoulder easily moves this way with every wing beat.

A bird can move this way because the orientation of its shoulder socket is directed upwards, as we learned in an earlier lesson.

Ancestors to birds had shoulder sockets allowing a greater range of motion than your shoulder, but less than a bird’s.