Cranial kinesis facilitates quick retraction of stuck woodpecker beaks

Abstract

ABSTRACT Much like nails that are hammered into wood, the beaks of woodpeckers regularly get stuck upon impact. A kinematic video analysis of pecking by black woodpeckers shows how they manage to quickly withdraw their beaks, revealing a two-phase pattern: first a few degrees of beak-tip-down rotation about the nasofrontal hinge causes the tip of the upper beak to be retruded while its proximal end is lifted. Next, the head is lifted, causing beak-tip-up rotation about the nasofrontal hinge while the lower beak starts retruding and initiates the final freeing. We hypothesise that these consecutive actions, taking place in about 0.05 s, facilitate beak retraction by exploiting the presumably low frictional resistance between the upper and lower beak keratin surfaces, allowing them to slide past each other. It also demonstrates the counter-intuitive value of maintaining cranial kinesis in a species adapted to deliver forceful impacts.