Hummingbirds Have an Easy Way to Fool Harassers

Male white-necked jacobins sport bright-blue feathers. So do the stealthiest females.

A female white-necked jacobin with a blue head
Irene Mendez Cruz / Cornell University

Jay Falk has some choice words for white-necked jacobins, the iridescent, blue-tinged hummingbirds he spent much of graduate school chasing through the Central American tropics. They’re “the show-off jerks of the hummingbird community,” he told me.

Falk, a biologist at the University of Washington, is deeply fond of the birds, who are gorgeous and clever and sassy. Sometimes, they’re brave enough to flit right up to him and inspect what he’s holding in his hand. But jacobins are also bullies, especially when they spot one of the species’ more modestly colored females, which sport green backs and mottled gray chests. These dull-feathered gals can’t even seek out a meal without being catcalled, pecked, or body slammed by their kin—acts that are sometimes about sex, sometimes about rudeness, and perhaps quite often about both.

But at some point in the birds’ evolution, it seems, a few of these females got fed up with being harassed. They jettisoned their traditional grayish-green garb, and instead donned the dazzling sapphire feathers that have long been linked to males—a costume change that can help some females skirt the jabs of their more brightly colored peers. The male masquerade cloaks the hummingbirds so well that they’re able to live out their days in relative peace, according to a new study by Falk and his colleagues.

“If you think about it, it’s kind of a nice solution,” Dai Shizuka, an ornithologist and evolutionary biologist at the University of Nebraska at Lincoln, who wasn’t involved in the study, told me. The birds’ stealthy strategy revamps one of the oldest themes around—if you can’t beat ’em, join ’em. But it’s also challenging some of the biases that human researchers may bring to their work, as they study the bodily traits and behaviors that make animals female or male.

In most birds, males are the showier sex: They’re the ones flashing their feathers, tussling for territory, and belting out serenades for their mates to assess. Females prefer their mates with glitz, but they have little reason to adorn themselves; gaudy feathers can make them more conspicuous to predators. “Ornamentation comes with risk,” Sarah Khalil, an ornithologist at Tulane University, told me. That white-necked jacobins, along with several other hummingbirds, buck this trend is “really odd,” Falk said. Only about a fifth of adult females manage the feat, but those that do are nearly indistinguishable from males, from the tops of their cerulean heads to the tips of their brilliant white tails. (Experts like Falk can differentiate the birds up close by inspecting very subtle traits or taking blood samples.)

Starting in 2015, Falk, then a graduate student at Cornell, began taking regular trips to Panama to watch the birds interact. He set up nectar-spouting feeders to attract them, and cameras to document them flitting back and forth. He spied them pinwheeling through the air, cavorting around one another, and executing Olympic-caliber flips mid-flight. But most of all, he saw them aggressively and relentlessly chasing each other. Grayer jacobins, he noticed, were almost always the quarry, with bluer birds in pursuit.

The pattern repeated when Falk stationed pairs of taxidermied jacobins at the feeders for live birds to inspect. Each day of the experiment, he offered the birds a choice. Sometimes, it was between a gray-green female and a blue male; other times, a gray-green female and a blue female; and still others, a blue female and a blue male. When both stuffed birds were vibrant, they received roughly equal amounts of attention. But whenever a modestly colored one was on display, it bore the brunt of the attacks. Some jacobins would strut up to the props and make obvious sexual advances—wiggling their butts, or diving at them while chirping—while others would batter them in an outright assault.

Falk couldn’t always discern the sex of the assailant, but it didn’t seem to matter much. When given the opportunity, a lot of jacobins walloped the grayer mounts with abandon. Even birds of other species sometimes took part. Being drably colored clearly gets you “beat up by all of these hummingbirds,” Teresa Feo, an ornithologist and evolutionary biologist at the Smithsonian’s National Museum of Natural History, told me.

Some of the targeting of grayish-plumed birds probably stems from sexual attention—males eager to mate with as many females as they can. But Falk thinks their woes hinge on at least one other powerful driver as well: hunger. Hummingbirds, with their fast-beating hearts and sky-high metabolic rates, are constantly in search of food, and often need to jostle others out of the way to reach it. But they also know to pick their fights, and the rowdy reputations of the bluest birds might precede them. Most jacobins “have probably been chased by one of those male-colored birds,” Falk told me. “So if you’re going to be aggressive, maybe you go for one that’s less likely to retaliate.” Another one of Falk’s experiments seems to back up this idea. The researchers found that blue females, tracked with ID tags, lingered longer at feeders than gray-green ones—a hint that the ruse was working. Dressing flamboyantly, it turns out, can be an excellent way to avoid the spotlight.

The findings mark the first in-depth investigation of male-mimicking females among wild hummingbirds, but the phenomenon isn’t all that rare. What’s happening with the jacobins has a distant parallel in damselflies, insects whose females pantomime the patterns of males to stave off excessive copulation. Many other classic examples run the opposite direction. Some male fairy wrens, for instance, will put off molting into their glammed-up adult forms so they can pass as females and avoid conflicts with their brethren, Tulane’s Khalil, who studies the birds, told me. The common thread in nearly all of these species “is male harassment, which is generally just a problem for everyone,” Shizuka, of the University of Nebraska, told me. Such torment is strong incentive to copy the sex that’s less likely to experience it.

Falk’s study is a reminder that, across species, males and females won’t necessarily have a signature, sex-specific look. “That’s the takeaway—you really can’t tell who is producing eggs or sperm,” Ambika Kamath, an evolutionary biologist at the University of Colorado at Boulder, who wasn’t involved in the study, told me. “I like that this [paper] takes weirdness seriously, which we should all do more often.” Maybe there’s also a better way to frame findings on avian color, Kamath said. The easiest term to describe certain females’ bright-blue plumage might be “malelike,” but even that could be a misnomer. Falk’s work showed that adolescent jacobins of both sexes are colorful, too, something unusual for a bird. “In a lot of cases, juvenile birds look similar to adult females,” until maturing males stray from the norm, Feo told me. Among jacobins, females are the ones striking out on their own.

Perhaps the big question on jacobin feathers is less about why females would want to resemble males, and more about why drab females “bother to look different,” Feo said. Despite all the torment they endure, they likely have something to gain from their grayness. Maybe it’s camouflage or maybe it’s more sex—researchers aren’t quite sure. Either way, their lives aren’t as devoid of color as they might seem.