The Science of Feathers
A bird's plumage is both mystifying and magnificent, and serves a plethora of purposes, from protecting it from the cold to attracting a mate.
Feathers are inherently what makes a bird a bird, yet their nature and origin have long mystified some of history's brightest minds, from Aristotle to Newton. Today's scientists are trying to better understand how and why jays glow so blue, parrots so green, flamingoes so pink. Using such high-tech tools as electron microscopy and high-speed video, they've been learning that feathers are multitaskers, carrying out multiple jobs simultaneously. They're flight surface, rain jacket, and courting outfit. They're a barometer of what a bird has been eating and how it has been living. In some cases they comprise a potent preventive medicine. Much of what we've always loved about this delicate medium through which birds meet the world is precisely what meets the eye--yet much of what we can learn to appreciate about them, it turns out, lies just a little beyond our normal range of vision.
A feather develops much like one of our hairs--a meticulously constructed mass of dead protein pushed out from a follicle in the living skin. But unlike a simple hair, a growing feather branches into a structure of fractal complexity. It's as if a tree were to rise not by developing an ever-more intricate system of branches and twigs, but rather by being pushed, as a wholly developed plant, straight up out of the ground.
Think of a feather as a treelike structure. The trunk: a hollow central shaft, which ornithologists call a rachis. The rachis sprouts numerous branches, called barbs. In many feathers, such as those that form the shape of the wings and tail, the barbs are then further subdivided into twigs, so to speak, called barbules. On flight feathers, the barbs all grow in the same plane, like an espaliered fruit tree tacked to a sunny wall. The barbules of adjoining barbs, meanwhile, hook closely together to form a smooth and remarkably stiff surface that's critical to maintaining a durable yet streamlined aerodynamic form. On down feathers, by contrast, the barbs twist willy-nilly in an ordered chaos that traps air and provides superb insulation.
The ingenious ways in which feathers perform so many functions simultaneously--insulating from cold; warding off the sun's ultraviolet rays; repelling water; making flight possible--have led ornithologists to vigorously debate how they evolved in the first place. The fossil record is increasingly rich with well-preserved feathers from dinosaurs and archaic birds. But because the earliest known feathers don't look much different than today's, paleontologists haven't been able to learn much about what caused them to evolve in the first place.
A few years ago a team of researchers took a close look at some fossil bird feathers preserved in 50 million-year-old slale deposits in Germany. An electron microscope revealed that the surface of some feathers was covered with something that looked like a squashed shag carpet--a landscape of tightly packed cylindrical blobs. Paleontologists had seen similar fossil structures before, but had interpreted them in a very different way. "There was speculation that these grains were bacteria," says Richard Prum, an ornithologist at Yale and a member of the research team. "But they were in fact melanosomes in feather cells."
A melanosome is a packet of dark pigment that exists within a cell and is, happily, a very durable object. This evidence showed that the fossil birds were colored--probably a dark hue with iridescent highlights, much like a modern-day starling or a blackbird. Using similar techniques, Prum and others have also been able to reconstruct some of the bright colors sported by feathered dinosaurs up to 150 million years ago.
One description of what a feather is, in other words, is that it is a physical means of encoding information, just like a printed page or a computer chip. And, as with what in our species passes for plumage--whether clothes, makeup, a Rolex, or an allegiance to workout videos--they likely developed long ago into the form we love today for that most fundamental of evolutionary reasons: sex.
Some feather colors--reds, oranges, yellows--result from pigments. Blues, on the other hand, are a product of intricate protein structures that reflect light in just the right way. (Greens tend to result from a combination of these features.) And iridescence, like that on a hummingbird's gorget, is another, more refined version of structural coloration in which proteins line up, like grooves in a compact disc, and all reflect light back in exactly the same direction.
A flamingo's pink is the result of the foods it eats, especially tiny crustaceans that it strains out of water and muck with its serrated bill. When flamingoes eat food containing certain colored carotenoid proteins, their digestive enzymes change those molecules into new pigments that are deposited in feathers. And those feathers, in turn, are replaced once a year during the annual molt.
In many species with pigmented coloration--that is, reds, oranges, and yellows--it's the more brightly colored male that gets the girl. For example, ecologist Kevin McGraw of Arizona State University has spent a lot of time studying the house finches that are common both in his state's desert environs and its urban settings. Male house finches show a wide diversity in color, from pale saffron to brilliant crimson. Why? Though the cause is partly genetic, color is also linked to diet. And it has repercussions.