Divergent nectar-feeding mechanisms evidenced by intralingual suction in sunbirds.

Nectarivory has independently evolved many times among birds, yet little is known about the diversity of feeding mechanisms that enable specialized taxa to efficiently collect this energy-rich resource. Multiple avian groups have converged on evolving elongated bills and tube-like tongues adapted for nectar extraction. Old World sunbirds (family Nectariniidae) stand out as having the greatest degree of convergence in bill and tongue morphology with the well-studied and highly specialized New World hummingbirds (family Trochilidae), which fill their tongues via fluid trapping and expansive filling. However, using museum specimens, high-speed video, and fluid modeling, we show that sunbirds use a unique drinking mechanism not found in any other animal: intralingual suction. Using tube-like tongues, sunbirds are able to move nectar from flowers to their mouths by generating a pressure differential along the length of the tongue, a remarkable feat for animals without lips or cheeks. We show how this feeding mechanism works and how it can be modeled mathematically. This discovery raises new questions for both vertebrate and invertebrate groups that feed on liquid food, and we anticipate that our results will be used to evaluate both food capture mechanisms and their efficiencies. For example, knowing the feeding mechanism allows the energy intake rate to be accurately predicted and tested, leading to reliable energy budget models and minimum requirements that, in turn, circumscribe more complex ecological interactions. VIDEO ABSTRACT.