HOMBERGER, D. G.; Louisiana State Univ., Baton Rouge: Function, ecology and evolution of two morphological bill types of Psittaciformes

The Psittaciformes (parrots and cockatoos) are considered a uniform avian orders based on their diagnostic features: (a) their curved beak with the upper bill tip projecting over the shorter lower bill, and (b) their prehensile foot. However, two fundamentally different bill types can be identified. The psittacid bill type (of the majority of Psittaciformes) has a projecting upper bill tip with a corrugated internal surface, a transverse step on the corneous palate, and a lower bill tip fashioned into a transverse cutting edge. It enables them to shell seeds by exerting pressure on preformed suture lines of seed coats with the mandibular cutting edge, which remains aligned with the upper bill. The use of the foot is not obligatory. In contrast, the calyptorhynchid bill type has a short upper bill tip apposed to the outer face of the lower bill, a lack of consistent surface structures on the corneous palate, and an emarginated lower bill tip with a pair of projecting lateral corners. It is typical of certain arboreal Black-Cockatoos (Calyptorhynchus lathami, Callocephalon fimbriatum, and certain C. banksii sspp.), which feed on seeds of often serotinous plants with woody fruits (e.g., Eucalyptus), whose plywood walls are part of a hydraulic mechanism for seed release in dry or hot conditions. They also extract insect larvae from branches and galls and tear through fibrous wood with a pincer-like bill action by deflecting the lower bill sideways to align one of its corners with the upper bill tip. The foot is not integral to this feeding mechanism. All Psittaciformes, however, possess a quadrato-mandibular jaw joint that enables them to move their lower bill tip from side to side. This feature indicates that the calyptorhynchid bill type is ancestral for Psittaciformes. (Supported by NSF)