Helmeted hornbill cranial kinesis: Balancing mobility and stability in a high-impact joint.

Prokinesis-in which a craniofacial joint allows the rostrum to move relative to the braincase-is thought to confer diverse advantages in birds, mostly for feeding. A craniofacial joint would, however, be a weak link if cranial stability is important. Paradoxically, we have identified a craniofacial joint in helmeted hornbills (Rhinoplax vigil), birds known for violent head-butting behavior.