Surface tension transport of prey by feeding shorebirds: The capillary ratchet

Authors: Manu Prakash, David Quéré, John W. M. Bush

Link: https://www.science.org/doi/10.1126/science.1156023

DOI: https://doi.org/10.1126/science.1156023

Abstract: The variability of bird beak morphology reflects diverse foraging strategies. One such feeding mechanism in shorebirds involves surface tension–induced transport of prey in millimetric droplets: By repeatedly opening and closing its beak in a tweezering motion, the bird moves the drop from the tip of its beak to its mouth in a stepwise ratcheting fashion. We have analyzed the subtle physical mechanism responsible for drop transport and demonstrated experimentally that the beak geometry and the dynamics of tweezering may be tuned to optimize transport efficiency. We also highlight the critical dependence of the capillary ratchet on the beak's wetting properties, thus making clear the vulnerability of capillary feeders to surface pollutants.

Additional Information:

Darwin was fascinated with bird beaks (amongst other things Darwin was fascinated with). Shape and form in biology (from the perfect beak of a bird to swimming paddle of a blue whale) evolve to optimize for function. But as with everything else in biology, it's often hard to tell how optimal is something (mathematically speaking). From an observation I made on a lake of a shore bird so fascinating (it catches your attention right away when you see it spinning in circles, all the time), I stumbled on an unusual strategy they use to transport fluids through the beak utilizing contact angle hysteresis. Usually contact angle hysteresis impedes motion, the reason why a drop of water sticks to window glass on a rainy day. Here was a case when this is the only reason the droplets are transported. We ended up calling this mode of transport a "capillary ratchet." The bird beak geometry is optimized in several species to take advantage of this physical principle. What else could have Darwin asked for - maybe a mathematical equation for his "finch" beaks.