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.