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.
"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.