The Survival Advantage of Being a Fancy Baby Coot
Unlike many other gaudy animal ornaments, the red-and-orange heads of coot chicks are honest indicators of weakness and vulnerability.
As adults, American coots have a drab color scheme, with black bodies and white bills. Their chicks, however, have an aesthetic that’s part drunk friar, part disheveled lion, and part tequila sunrise. Their faces and bald pates are bright red, while their necks are encircled in scruffy yellow-orange plumes.
These garish colors are very strange. Most bird chicks come in dull, camouflaged hues. And while nature is full of animals with elaborate, conspicuous ornamental traits, from the resplendent tails of peacocks to the branching antlers of deer, many of these traits are about sex. They make their bearers more attractive to mates, either because they’re sexy in their own right or because they’re honest signs of health and vigor. So why on Earth would a baby bird be so fancy? It clearly has nothing to do with sex. And as Bruce Lyon and Daizaburo Shizuka from the University of California at Santa Cruz have shown, the garish colors aren’t signs of quality chicks either. They’re the opposite.
The life of a coot chick can be brief and brutal. The parent birds lay six to 12 eggs, but almost always more than they can actually raise. The eggs hatch one by one over the span of a week, and the resulting chicks scramble and compete for their parents’ food and attention. It’s an unforgiving hunger game, which the youngest, smallest ones typically lose, at the cost of their life. About half of the chicks die before they’re a week old. This seemingly wasteful system makes sense for parent coots. They overproduce chicks on the off chance that they can procure a glut of food and raise a larger-than-average family. If they can’t, the surplus chicks will just die off—no harm, no fowl.
After the first week, when the weakest chicks are all dead, the parents change their behavior. Each now picks a favorite among the survivors, and provides that chick with 80 percent of the food it collects. These golden children grow rapidly, while their unchosen siblings are grabbed by the head, vigorously shaken, and chased away. In the 1990s, Lyon learned that parents pick their favorites in part because of their gaudy plumage. By trimming the orange feathers around the chicks’ necks, he showed that the more ornamented chicks got more food and grew faster than their siblings.
Over time, Shizuka and Lyon were able to identify other important patterns by studying more than 1,400 chicks across hundreds of clutches, and that work finally helped explain the red and orange colors. They saw that parents also care about age when picking favorites. Once their initial clutch has been culled to an affordable size, they tend to focus on the youngest remaining chicks, while violently neglecting the oldest ones. At this point, the oldest chicks are big enough to start finding their own food, but the younger ones are smaller and will starve on their own. The parents reduce that inequality by feeding the heck out of their youngest chicks and getting them to the same level as their siblings.
Crucially, the youngest chicks also tend to be the most colorful ones. The colors come from carotenoid pigments that the mothers add to their yolks, and they seem to dump more of these pigments the more eggs they lay. In this way, the mother effectively paints her chicks by order of age. Once they’ve hatched, she and her mate can then work out which ones are the youngest and the most likely to need their help. “Bright chick colors appear to act as a signal to parents, prompting them to play favorites to the offspring that would benefit most from extra food,” explains Mary Caswell Stoddard, an ornithologist at Princeton who studies bird colors.
If this is right, then the red-and-orange heads of coot chicks are very different from other kinds of animal ornaments. The vividness of such traits often reflects an individual’s strength and vigor. But the brightness of a baby coot’s colors instead reflects its weakness. It’s an honest indicator of vulnerability.
“What is exciting about this comprehensive study is that it raises so many new questions,” Stoddard says. How exactly do mothers influence the colors of their chicks, and do they have any control over that process? And why aren’t coots better at gaming this system?
They certainly try. Coots are prolific “brood parasites,” which means that they will lay their eggs in the nests of other coots, offloading their parental duties onto their unwitting neighbors. About 40 percent of nests contain at least one “parasitic” egg that doesn’t actually belong to the local parents.
But this strategy usually fails. For whatever reason, coots tend to drop parasitic eggs early in the laying season, which means that the intrusive chicks are less bright than their foster siblings, and less likely to be chosen as favorites. The parents can also tell that they don’t belong. As long as the first chick that hatches is actually theirs, they can use this image to distinguish their own babies from a neighbor’s, which they then murder.
“It’s a low-success strategy, but if you get a neighbor to raise your chick, why not try?” Shizuka says. “Whatever success you get is a free chick.” But why don’t the parasitic parents switch up their laying order and laden their neighbors with the latest eggs, which will produce the most vulnerable-looking chicks? It’s not clear. It’s also unclear if other animals use similar badges of vulnerability.
“We’d like to think that this is not just about coots,” Shizuka says. For example, many bird chicks have colorful patterns inside their mouths, which are visible to the parents when they beg for food, and might also act as badges of need. Shizuka also notes that vulnerability signals don’t have to be visual ones. “The coots have evolved a form that we can see and is apparent to us,” he says. “But really, this is about how babies convince their parents to invest in them. Other species could do this through vocalizations or sensory modalities that we don’t see as easily.”