Waders consume prey in droplets lifted from the water.
Birds can beat gravity in ways other than flying — some waders can also make food travel upwards by quickly opening and closing their beaks.
To investigate how this could happen mathematician John Bush from the Massachusetts Institute of Technology, Cambridge, and his colleagues watched a range of wading birds with long, thin beaks.
To catch prey in open water, the birds swim quickly round in tight circles to create a vortex that drags small insects and crustaceans towards the surface. With beaks unsuited to sucking, the birds instead peck repeatedly at the water surface while opening and closing their beaks. This opening and closing causes droplets, and their contents, to travel up their beak into the bird's mouth, in an anti-gravity conveyor belt.
Bush and his team created a stainless-steel beak to recreate the birds' feeding motion. They found that the food-containing droplet can defy gravity thanks to slight changes in the water’s surface tension when it is in contact with the beak. “It’s a little bit subtle,” says Bush. The results are published in Science1.
Small water droplets, when in contact with a surface, are dominated by a phenomenon called contact-angle hysteresis. The uneven nature of surfaces means that the angle that the drop makes with the surface is not the same everywhere. As a drop grows in size, the angle increases until a critical point known as the advancing angle is reached.
At that point the drop starts to move to accommodate the extra liquid. Likewise, if some liquid is taken out, at some point a critical angle, the receding angle, is reached, and the drop will move up the beak to overcome this. The net force created by these different angles is what holds a raindrop in place on a vertical window pane.
When the waders in Bush's study close their beaks, the droplet is squeezed until the advancing angle is reached in the water at the beak's tip (see video). The leading edge of the droplet moves up the beak, but the other end stays put. Likewise, when the beak is opened slightly, the trailing end of the droplet at the widest part of the beak reaches the receding angle and shifts up the beak while the other end stays still.
It works best for beaks measuring between 2 and 8 centimetres in length, the researchers add.
Tweezey does it
“When the beak is moved in this tweezing fashion, the drop inches up slowly, one side at a time,” explains Bush. “There is only a very fine range of angles for which it works,” he adds.
This ratchet feeding mechanism depends on the surface of the beak, and so birds that rely solely on this mechanism for feeding could be at risk if the birds come into contact with oil spills, says Bush: “If you drop the beak in oil these birds can’t feed.”
Improved knowledge of surface tension and contact angles could lead to self-cleaning windows, fluids that can slip unhindered through tiny pipes or even surfaces that are sticky to liquids in one direction but not in the other.
Prakash, M., Quéré, D. & Bush, J. W. M. Science 320, 931–934 (2008).