Is hummingbird's feeding mechanisms based on elasto-capillary ?

Hummingbirds have a high metabolic rate requiring to feed on a very energetic resource: the nectar. For this purpose, they have developed a truly specialized nectar-feeding method. During the capture of the nectar, the hummingbird’s bill goes into the corolla of the flower. Then the tongue is protruded out and plunges into the nectar. After the loading, the tongue retracts and is unloaded in the bill. The mechanism of the tongue’s loading is still unresolved. At first, it was suggested that the tongue was composed of two thin tubes which was filled because of a capillary suction mechanism (Martin 1833, Scharnke 1931). But the morphological study of the tongue reveals that it is composed of two folded sheets and no closed tubes. In this talk, we will tackle the question: how a sheet can fold into a tube to capture viscous fluid. Several mechanisms were proposed: self-assembling capillary syphon (W. Kim et al. 2012) or elastic micropumps (A. R. Guevara et al. 2005). Based on elasto-capillary experiments and recording of living hummingbirds, the relevance of both models will be discussed.