wing bone construct

Misconception 1: "Birds can extend their arm bones into a straight line like humans."



figure 1

Looking at the underside of the outstretched wing (figure 1) as it comes into land, reveals the Humerus, Radius and Ulnar (the arm bones between the shoulder and the wrist) of the wing. Notice the angle of the two lines at the elbow, given the wing is near to fully extended. You would expect this angle to be around 180° and the arm to be fully extended. This is not the case (figure 2), in fact the angle at the elbow is nearer 80°.

figure 2

Full extension of the arm is prevented by the Patagial Tendon (figure 3) which is essentially a chord that attaches from shoulder to wrist preventing the joints from hyper extending beyond 80°. As the elbow supinates towards the body the tendon forms an arc between the shoulder and wrist forming that iconic heart-shaped silhouette (figure 4) we associate with birds.