Bird beaks are more than nature's ingenious designs. They're wonders of evolution, barometers of how species react to--and sometimes even instigate--changes in their environment.
Finches with their hefty seed-crackers; warblers with their forceps made slender for extracting small insects hidden among leaves and stems; raptors with their curved hooks for tearing; shorebirds with their probes, straight or curved, which help them extract foods buried on a beach or mudflat. Novice birders quickly learn that the wild diversity of bird beaks is among the most reliable means of quickly determining to what family, and often even what species, a bird belongs. When you're faced with the bewildering array of avian life in a fall marsh or spring woodlot, that certitude is a comfort, something solid to rest on.
But it's a bit misleading, too. Birds' beaks are, in fact, always changing. They're not static over the course of an individual bird's lifetime, and they're certainly not fixed as bird species respond to--and instigate--changes in their environment. Yes, the basic order taught in Birding 101 is there. But scientists have come to learn in recent years that bills are far from being blunt instruments. Rather, they're delicate barometers of their surroundings. To examine in detail how they work is to be transported from simple satisfaction at the intricacies of efficient natural design to wonderment that evolution can get things so precisely right in so many ways.
"Birds have had this explosion of variety in their beaks," says Margaret Rubega, a biologist at the University of Connecticut. "Most vertebrate animals don't have nearly as much variation in how their jaws work. But at every stage of the process of changing beak shape it has to work; at every stage in their evolution birds had to feed themselves successfully. The bird that ends up not fed ends up dead."
Rubega began studying red-necked phalaropes in the early 1990s. Phalaropes are the black sheep of the sandpiper family: Females are larger and more colorful than males, and they take multiple mates. They feed in a distinctive manner, too, spinning like dervishes on the surface of a lake or ocean to concentrate the tiny crustaceans and other aquatic invertebrates they like to eat in a column of water, then grabbing the organisms with their long, straight bills. But the tip of a phalarope's bill is a long way from its mouth. So how does the prey make that journey? The birds don't lift their heads, which means whatever they're ingesting has to be elevated against the force of gravity. And beaks are not formed like straws, so suction can't explain what happens.
Using high-speed videography, Rubega was able to reveal what does occur. A feeding phalarope swiftly opens its bill after grabbing its prey. The food, embedded in a water droplet, races up between the bird's jaws, often in as little as one two-hundredth of a second. How does it do that? Through surface tension, Rubega found--the same force that causes water to bead on a window. Water molecules are attracted both to one another and to the molecules lining a bird's bill. As a phalarope opens and closes its bill, the molecules move closer together--which has the result of pulling the droplet up into the bird's mouth. (Recently, MIT researchers built a model phalarope bill, providing additional support for Rubega's explanation, and coined the term "capillary ratchet" for the bird's feat of biomechanical wizardry.)
Rubega observed that some birds, the star performers of the phalarope world, open their bills only a single time to raise a droplet; others need two or three tries. By looking at cross-section anatomy measurements of the birds' beaks, she was able to correlate the efficiency of a bird's feeding with its anatomy. Phalaropes have a complex series of humps on the inside of their upper jaws that increase the effect of surface tension because they give water molecules more surface area to hang onto; the more complex this topography is, the more efficiently a phalarope feeds.
"When you look at the outside of the beak, which is what scientists generally measure, there's no relationship to how well they do this transport, because that's not what makes the difference," she says. "All that matters is the internal dimensions of their beak."
One of Rubega's graduate students, Gregor Yanega, photographed hummingbirds in action to learn how these long-billed birds, so well evolved to feed on nectar, manage to capture the insect prey whose protein they also need. The question was, as Rubega puts it, "How do they manage to snatch insects with that long, delicate tool? It's as if they're using a set of chopsticks instead of a catcher's mitt."
To their surprise, Yanega's high-speed videos showed hummingbirds bending their jaws out to the sides in order to dramatically increase the size of their gape and angle the far end down to get it out of the way. In other words, they move their chopsticks out of the way in order to make better use of the catcher's mitt that lies beyond.
"We were gobsmacked," says Rubega. "We had to play the initial piece of tape back three or four times to convince ourselves that was really what we were seeing."