surface structure beak The avian beak is an external anatomical structure, in the head region, that is adapted for feeding self and young, catching prey, probing, etc. It encompasses, but is not restricted to, the maxilla, mandible, maxillary ramaphotheca, mandibular ramaphotheca, nostril, nasal fossa, nasal bones, egg tooth and rictus[GO]. The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, probing for food, courtship and feeding young. avian beak rostrum bill