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.
"More color appears to lead to better competitive ability," McGraw says. "Yellow males are more dominant and aggressive, but red males do better in reproduction. Females consistently prefer more colorful males. The molt period when finches are growing their feathers is amazingly costly. So it's a perfect time for an animal to display how good it is through its color."
A recent study from wetlands in southern Spain, though, has shown that the breeding color of flamingoes is due not solely to that once-a-year event. Rather, it's strongly connected to the color of the oil that these waders produce from their preen gland. This small organ, located beneath the tail, produces oil that birds carefully transfer to their feathers by rubbing themselves with their bills and heads. The oil is an important tool in maintaining the health of feathers, as it helps make them water repellent and also wards off lice and other feather parasites.
The preen oil of flamingoes, as it happens, contains the same pink pigments as flamingo feathers. A team of Spanish researchers recently showed that male flamingoes, perhaps more akin to some human beachgoers than we might like to admit, spend a lot more time rubbing themselves with this oil during the breeding season than at other times of year. As a result, they grow pinker, and presumably more attractive to potential mates, and they begin nesting earlier.
ATTRACTING MATES #2
In Europe female barn swallows prefer males with the longest tail streamers; in North America they preferentially choose males with darker breast feathers. Ecologists have verified this through not only observation but experimentation. If you intensify the color of an American male's breast feathers during the breeding season, he will on average breed earlier and father more young. McGraw, Rebecca Safran of the University of Colorado, and some colleagues tried that experiment a few years ago. But in a more recent study they found something more remarkable: males altered in this way aren't more successful only because their appearance changes how other birds interact with them. Rather, they experience a surge in testosterone. And the reverse is true, too: color a female barn swallow's breast feathers, and her testosterone level drops. She effectively becomes more female.
A bird's external appearance, in other words, can change its internal hormone levels. If clothes make the man, then feathers really do make the bird; the information that's encoded in them doesn't just speak to others, but to their wearer. How does this happen, given that birds don't use mirrors or cameras? Probably through social feedback, Safran says--it's much the same dynamic that causes male football fans to experience a surge in testosterone when their team is winning. In this case, changing the appearance of a barn swallow alters how neighbors and potential mates act toward it.
"What we've shown is that you can change an individual's appearance and its physiology catches up," she says. "It's a very dynamic system. A difference in appearance leads to them being treated differently by other members of the group. And that behavior really feeds back to an individual's hormones."
A feather represents an investment of a year or more on which a bird stakes its life. Of necessity, a bird tries to keep it in good shape. The physical act of preening helps keep lice in check. Preen oil keeps feathers supple, and in many species helps assure water repellency. And some birds go even further than that. A few years ago Edward Burtt of Ohio Wesleyan University, working with McGraw and some other researchers, learned that colorful pigments found in some parrot feathers have antibacterial properties. When feathers containing these unique chemicals are exposed to feather-degrading bacteria, they deteriorate more slowly than white feathers that lack pigments. Such antimicrobial qualities, the researchers speculate, may have evolved specifically to deal with the challenges of the places most parrots live.
"As you get to moist tropical environments, bacteria pose a big challenge to feathers," McGraw says. "It's a constant battle. And so parrots have developed unique feathers with pigments that are found nowhere else."
Yet in all birds a feather's autumn inevitably arrives. It falls out of its follicle and the follicle begins to push out a replacement feather. That sounds simple; we do the same thing all the time with hairs. But birds rely on their feathers so much that molting is inevitably a big deal. It's energetically costly, which is why female birds are impressed by males who can afford lushly colorful reds and pinks when they grow new feathers. And it's tricky to do without plumage that is, after all, critical to its wearer's survival.