Kim Wonjung and his colleagues at MIT, in the US, have worked out how the tongue of the hummingbird can achieve a capillary action. They publish their paper, The hummingbird's tongue: a self-assembling capillary syphon, in the Proceedings of the Royal Society B: Biological Sciences today.
The ruby-throated hummingbird uses an elasto-capillary action to suck up its nectar without great resistance. Surface tension drives nectar automatically up the grooves of the tongue, which is then withdrawn and unloaded. After the tongue is taken out of the nectar, it changes shape completely and traps the liquid in its grooves. It has been regarded as a kind of fluid trap, with retraction of the tongue moving the nectar towards the mouth.
Filming the captive bird with a 10% sucrose solution, the tongue was extended 13mm from its bill. Instant capillary rise into the tongue was observed through the glass in a literally clear demonstration. At high magnification, the tongue tips stick together at first, probably due to pre-wetting. The fluid causes separation of the tips then it starts to rise at 20cm s-1.
Loaded up, the flexible tongue will then unload after it has re-entered the bill. The bird does this by squeezing the tongue between its upper and lower bills. The flexibility is useful both in entering bent nectarines of flowers such as jewelweed and also to unload nectar in the mouth.
The hummingbird (Archilochus colubris) drinking from a transparent feeder; Credit: © Wonjung Kim, Franc¸ois Peaudecerf, Maude W. Baldwin, and John W. M. Bush
You may have used a capillary siphon for various transfers of liquids, but this action by humming birds uses an incredible self-assembling capillary siphon. Because nectar is stored in shallow, small scale areas in flowers, the tongue is not under the surface but rather adopts capillary action as its preferred method of taking out the "juice."
At a 150° angle of tongue, the uptake of nectar and therefore its energy uptake is at the max. With co-evolution from its preferred flowers the flexible tongue and its bill have adopted the semi-circular cross section that is also found in sunbirds and honeyeaters. Parallel evolution could well have come into play too, but personally I love the way these common little acrobats enliven our gardens. Outside of the Americas, such sights are rare indeed.
The male with the ruby throat, showing off, but Earth Times always has a kind of love affair with these animals, such as Anna's hummingbird - Male Ruby-Throated Hummingbird Image; Credit: © Shutterstock