Functional morphology of hummingbird bill tips: their function as tongue wringers

Nectarivores are animals that have evolved adaptations to efficiently exploit floral nectar as the main source of energy in their diet. It is well known that hummingbirds can extract nectar with impressive speed from flowers. However, despite decades of study on nectar intake rates, the mechanism by which feeding is ultimately achieved - the release of nectar from the tongue so that it can pass into the throat and be ingested - has not been elucidated. By using microCT scanning and macro high-speed videography we scrutinized the morphology and function of hummingbird bill tips, looking for answers about the nectar offloading process. We found near the bill tip, in an area of strong lateral compression of internal mandibular width, that the tomia (cutting edges of the bill) are thinner, partially inrolled, and hold forward-directed serrations. Aligned with these structures, a prominent pronglike structure projects upward and forward from the internal mandibular keel. Distal to this mandibular prong, another smaller maxillary prong protrudes downwards from the keel of the palate. Four shallow basins occur at the base of the mandibular prong on the mandibular floor. Of these, two are small basins located proximally and at the sides of the mandibular prong. A third, slightly larger basin is positioned distally to the first two and directly under the maxillary prong. And the fourth basin, the largest, is found more proximally where the bill becomes thicker, as seen from the side. We documented that this group of structures is integrated into the area of the bill where tongue extrusion occurs, and we hypothesize that they function to enhance the nectar release at each lick. We suggest that this "wringer", operated by bill and tongue movements, helps to move nectar towards the throat.