A Bird’s Stinky Egg Is Actually a Humblebrag

Female hoopoes will paint their eggs with a brownish goo, in a bid for greater male attention.

a hoopoe bird perched on a branch
David López / EyeEm / Getty

In preparation for birth, some expectant human parents will build IKEA cribs or down prenatal vitamins. Female hoopoes douse their eggs in a pungent postcoital goo.

Shortly after laying their eggs, these delightfully zebra-striped birds will begin to paint the clutch with their beak. The pigment they use is made in-house—a brown, oily substance secreted by the uropygial gland, located at the base of the female’s tail. The eggshells start out cerulean, but with each coat of fluid, they transform into a mucked-up greenish-gray. Though subtle in appearance, the secretions are rank: Thanks to the bonanza of bacteria within, they reek like “a very strong and smelly cheese,” tinged with a putrid je ne sais quoi, Juan José Soler, a biologist at the Experimental Station of Arid Zones, in Spain, who has spent years working with the birds, told me. The first time Soler grew the malodorous microbes in the lab, they stank up his entire department.

Humans might recoil, but hoopoes don’t mind the stuff. They dollop it liberally into their eggs’ sponge-like pores, imbuing them with friendly microbes that protect the eventual hatchlings from pathogens—a sort of living antibiotic. The grodier and grayer the shell, the more likely the embryo within is to survive—a trend that male birds appear to have picked up on. In a paper published today in Proceedings of the Royal Society B, Soler and his colleagues at the University of Granada report that male hoopoes will bring more food to females tending less color-saturated eggs, an apparent attempt to invest in higher-quality chicks. It’s a heartwarming display of fatherhood, spurred on by the intergenerational application of a stinky cosmetic.

The behavior, which has never before been observed in birds, is as notable as it is bizarre. Most documented signals exchanged between male and female birds center on hot-to-trot bachelors flashing their most appealing traits, such as dazzling plumage or nest-building prowess, to coax lady birds into having sex with them. This newly discovered hoopoe trick turns that narrative on its head. Here, females are doing the advertising, after the copulatory deed has already been done. Sexual scrutiny, it seems, doesn’t stop even after birds couple up.

“Mate choice is a multistep process,” Sahas Barve, an evolutionary ecologist at the Smithsonian National Museum of Natural History, who wasn’t involved in the study, told me. “Just like humans, birds are constantly judging their partners to make sure they made the best choice.”

The strange amorous antics of hoopoes—named for their distinctive whooping cry—have long been known to science. During their unusually long breeding season, which roughly spans from February to July, males adopt a variety of strategies to woo potential mates, including seductive serenades and mid-flight beak jousts with competitors that can leave losers blinded. Eventually, males and females pair up, mostly monogamously, spawning about half a dozen eggs that mothers incubate while their mates ferry in insect meals. For a couple of weeks or so, the female sits tirelessly on her clutch; what the male brings her is pretty much the only food she gets.

Raising kids is hard, and after hoopoe chicks hatch, things can get a little nasty in the nest. Within days of emerging, nestlings learn to defend themselves by hoisting their butts skyward and squirting liquid streams of feces at potential intruders. Silvia Díaz Lora, one of the study’s authors, calls this behavior “shooting the shit,” and has learned to open hoopoe nest boxes only while armed with plastic bags to catch the initial spray.

What residue gets left behind isn’t always cleaned up by mom. But the females’ uropygial secretions might constitute a preemptive counteraction to domestic slovenliness. Within hours of laying their first eggs, the birds will start dribbling the fluids over the shells with their needlelike beaks, one painstaking droplet at a time. They repeat the process over days, until the shells’ pores have been filled and the eggs’ color is largely uniform. (Most birds use similar fluids to keep their feathers clean and healthy; the mom’s preened bellies might also help work the oils onto the eggs’ surface.) This labor of love buoys the eggs’ shot at successfully hatching. But not all shells will end up looking the same, inviting some sharp-eyed judgments from males.

In 2014, a team led by Soler and Manuel Martín Vivaldi, also of the University of Granada, found a link between the color of uropygially painted eggs and antimicrobial activity. Eggs that were grayer—that is, less intensely colored—boasted more beneficial bacteria, and were thus better guarded from infection. This pattern of microbial protection is hard to discern with crummy human eyes. But bird vision is keen, capable of distinguishing “very subtle differences between colors,” Javier Medel Hidalgo, a visual ecologist at the University of Exeter, in England, told me. The University of Granada scientists hypothesized that male hoopoes might be scoping out the eggs to see which ones were über-sanitized, and spent the next several years trying to prove it.

Their latest study, which followed the egg-smearing shenanigans of hoopoes tending 61 nests, seems to nail the connection. Díaz Lora and her colleagues waited for females to paint their newly laid eggs in a variety of hues, then switched the contents of several clutches to help ensure that the team was measuring males’ response to color alone. Having rejiggered the nests, they tracked how much food males trucked in to each female. As the researchers expected, grayer sets of eggs, on average, garnered more grub for mom. The differences in the amount of food weren’t massive, and an oddly colored clutch probably wouldn’t turn a doting father into a deadbeat dad. But during this crucial time, egg painting might help make life cushier for certain hoopoes.

As far as the researchers can tell, the color-coded system is an honest one: A female hoopoe can’t paint her eggs deceptively to cheat extra attention out of her mate. But egg color might act both as confirmation that the males’ choice of partner was a good one, and incentive to up their commitment further. If male hoopoes are actually cluing in to this signal, and acting accordingly, they’ll increase their chances of yielding big, successful clutches and passing on their genes, Julie Hagelin, an ornithologist and a behavioral ecologist at the University of Alaska Fairbanks, who wasn’t involved in the study, told me. In evolutionary terms, that’s akin to “winning the lottery.”

Other birds, including robins and starlings, have been spotted titrating their investment in their kids based on egg colors. But egg complexion is usually an inborn trait; hoopoes are unusual in their ability to alter those hues after the shell’s taken shape. Martín Vivaldi told me that these findings mark the first time cosmetically colored eggs—wearing the avian equivalent of makeup—have been shown to goad males into stepping up their parenting game.

Given the unmistakable odor of hoopoe secretions, Hagelin told me she can’t help but wonder whether males are using their sense of smell to gauge antimicrobial potency as well. In Germany, it’s considered an insult to “stinken wie ein Wiedehopf”—to stink like a hoopoe. But every species has its own way of sniffing out a good mate.