Then there's the vast spectrum of plumage in the bird world, a riot of brilliantly hued buntings and carnival-colored parrots; the vibrant Palawan peacock-pheasant, its glossy blue-black feathers lustered with a dazzling metallic green; the red bird-of-paradise, with its filmy plumes and long plastic-like feather wires projecting from its tail, and its cousin, the paradise riflebird, with its outlandish super-black feathers created by unusual bristling microstructures that trap nearly all light; as well as the whiskered auklet of the Aleutian Islands, which sprouts acutely sensitive plumes from its head that guide it through its dark nest cavities in the nesting season.
James Dale studies color in birds and how they use it. "Birds can't use their color as a weapon, but they can use it to avoid conflict," he says. An ornithologist from New Zealand (land of the bright purple pukeko), Dale has devoted his career to making sense of the fantastic variety. There are some rules, he told me. Three in particular: Males are flashier than females, which are often a dull color so that they blend in with their surroundings while they're incubating eggs. Adults are more colorful than youngsters. Birds are brighter in the breeding season.
"But birds are rule breakers," he says. To name a few renegades: Female red phalaropes and painted snipes are more colorful by far than the muted males of the species. American coot chicks, with their bright red beaks and caps, outshine their dull parents—and for very good reason. Parent coots preferentially feed ornamented chicks over non-ornamented chicks. In male red-backed fairy-wrens, it's the social environment that determines whether young males molt into their flashy red-black breeding plumage—specifically, whether there are any old males around to harass the young birds and drive them away.
Perhaps chief among the color rebels is a parrot that lives in remote areas of northern Australia and New Guinea, Eclectusroratus (from the same Greek root as eclectic, and roratus for the sheen on the bird's plumage).
"Few birds have puzzled scientists more than this parrot," says Robert Heinsohn, a professor of evolutionary and conservation biology at Australian National University who studied the bird for nearly a decade. Heinsohn reports that when the great evolutionary biologist William Hamilton gave lectures, he would show an image of a male and female eclectus sitting together. The male was a bright grassy green and the female a resplendent crimson, her belly "bedewed with a blue haze," as the bird's European discoverer described it—in stark contrast to the normal pattern in dimorphic birds, where females are drab and males are brightly colored. "No other bird has both sexes so 'beautified' in different ways," says Heinsohn. In fact, so flamboyant is the female's plumage and so distinct from the male's that for the first hundred years after the parrots were discovered, people thought they were separate species. "Then one day," says Heinsohn "some naturalist saw a green one on top of a red one."
In a handful of other species, females sport brighter, fancier plumage than males. These include phalaropes, spotted sandpipers, painted snipes, wattled jacanas, and button quail. But in each of these cases, there is a reversal of usual sex roles, with males incubating eggs and females defending territories and fighting among themselves for access to males. "So these species are really the exceptions that prove the rule because they demonstrate that the competitive sex is the one most likely to have bright colors," says Heinsohn.
Not so among the radical eclectus parrots. There's no role reversal here: Females incubate eggs and raise their young. Moreover, even the chicks are rule busters. Unlike the chicks of most birds, which hang on to their drab unisexual juvenile plumage for at least the first year of their life, eclectus chicks hatch with sex-specific down colors and then molt straight into the dramatic full-color adult plumage.
According to Heinsohn, William Hamilton ended his lecture featuring the parrots with the line "When I understand why one sex is red and the other green, I will be ready to die." Sadly, Hamilton died of malaria contracted on an expedition to the Congo before Heinsohn unraveled this mystery—and another, perhaps even stranger, puzzle that's closely knit to it.
If the feathers of eclectus parrots are odd, their breeding conduct is even stranger. Female eclectus parrots are known to kill their own sons as soon as they hatch. This is one of those behavioral riddles so counter-intuitive it boggles the mind.
From a biological point of view, it's easier to understand infanticide when it involves killing the young of others for food or other competitive reasons. But killing your own offspring? Producing young is so energetically expensive that producing them and then promptly doing away with them makes little biological sense.
It's even harder to grasp why a parent would systematically kill off just one sex. This sort of sex-specific infanticide is extremely rare in the animal world. Aside from the wasted effort, it leads to inequities in the sex ratio of a population: too many females competing for too few male mates or the other way around. But as Heinsohn discovered over ten years of research in remote northern Australia, mother eclectus birds sometimes target the removal of their male nestlings within three days of hatching. Heinsohn often found the chicks at the base of a nest tree, pecked to death.
Why would a mother kill her sons? What drives a bird to such extreme behavior? And what possible value could it have to her reproductive survival?