Friday, 25 September 2015

How do hummingbirds drink nectar? And what about sunbirds?

The recent paper Hummingbird tongues are elastic micropumps published in Proceedings of the Royal Society has attracted attention in news media throughout the world.

The long-held notion that nectar flows by capillarity into grooves in the tongue that effectively form cylinders along part of its length seemed, first of all, to be supported by photographic studies and theoretical calculations. That paper was published, also in Proceedings of the Royal Society, in 2012 and entitled, The hummingbird’s tongue: a self-assembling capillary syphon.

Results in the most recent paper by Alejandro Rico-Guevara, Tai-Hsi Fan and Margaret Rubega were obtained by using high-speed cameras in eighteen species. It appears that the grooves, empty by compression inside the bill during protrusion and remain compressed until the tips of the tongue contact the nectar. The grooves then expand and fill completely with nectar. The tongue is withdrawn into the bill and the process is repeated. In other words, the observations are compatible with the mechanism being an elastic pump. With capillarity, these authors argued, the grooves should open to form cylinders before the tip of the tongue touches the nectar and a fluid meniscus should be apparent in the grooves. Those properties were not seen.


A video from the University of Connecticut (where two of the authors of the 2015 paper are based) can be seen here:


By contrast, the authors of the 2012 paper did report a meniscus in one species of hummingbird feeding from an artificial feeder, and the question is whether some difference in methodology or condition of the birds was responsible for the variation seen in the high-speed videos taken by the two groups. Two videos from that paper are shown here and here.

I suspect we have not heard the last of this story. It may not be one of those cases of either/or but of both, depending on circumstances as yet unknown. I can only add the observation that leaving things to capillarity does seem somewhat passive. In birds with such a high metabolic rate would it not be expected that an energy efficient pumping mechanism, faster than capillarity, would offer a selective advantage?

Finally, of course, the question must be asked: what is the mechanism in sunbirds, those unrelated, non-hovering nectarivores, of the Old World? Similar rates of nectar removal from flowers and artificial feeders to those recorded in hummingbirds have been reported*.

A female White-breasted Sunbird (Nectarinia talatala) that lived
in my office and at home at weekendsfor several years before
it went to join a male. It was fed exclusively on an artificial
nectar I designed specially for sunbirds

As a postscript it is worth noting that the titles of scientific papers are phrased very differently indeed. This is Gadow’s title for a paper on the subject that appeared in Proceedings of the Zoological Society of London in 1883 (pp 62-69): On the suctorial apparatus of the Tenuirostres.

It must be time to apply my suctorial apparatus to my dinner.

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†Kim W, Peaudecerf F, Baldwin MW, Bush JWM. 2012. The hummingbird’s tongue: a self-assembling capillary syphon. Proceedings of the Royal Society B 279 4990-4996.
‡Rico-Guevara A, Fan T-H, Rubega MA. 2015. Hummingbird tongues are elastic micropumps. Proceedings of the Royal Society B 282 20151014.
*Paton DC, Collins BG. 2006. Bills and tongues of nectar-feeding birds: A review of morphology, function and performance, with intercontinental comparisons. Australian Journal of Ecology 14 473-506.