Invisible work is no stranger to an adult female Jacobin hummingbird.
When she lays an egg, the male hummingbird, which played an equal role in conception, is nowhere to be seen. It is only thanks to their hours of weaving that the egg has a nest at all. When her chick hatches, she feeds it alone with vomited food from her long beak.
And then there is the constant harassment. While the covered-green females visit flowers to sip nectar, they are hunted, pecked and beaten with the body by aggressive males of their kind, whose heads are an extravagant blue.
But some female white-necked Jacobins, found from Mexico to Brazil, have a trick in their wings: instead of dressing in green plumage, they adopt bright blue ornaments and appear essentially identical to male hummingbirds. Scientists found these male doppelgangers avoided harassment against green women, according to an article published Thursday in Current Biology magazine.
For the past 50 years, most scientists have relied on the theory of sexual selection or mate choice to explain why so many male birds have such flashy features, like the mirage of a peacock’s tail feathers or the sapphire blue head of a hummingbird, said Jay Falk, a postdoctoral fellow at the University of Washington and author of the article. Dr. Falk led research on the paper as a PhD student at the Cornell Lab of Ornithology and while working at the Smithsonian Tropical Research Institute.
But these theories can disintegrate when applied to female birds, which can develop their own ornaments for evolutionary benefit unrelated to finding male mates.
“If we focus too much on men and sexual selection, we inevitably miss the big picture and not a comprehensive view of nature,” said Dr. Falk. In his view, the antidote is social selection: a theory that regards the social life of the entire species as the driving force behind evolution.
“The assumption that these extravagant traits have something to do with sexual selection needs to be tested,” said Kimberly Rosvall, a biologist at Indiana University, Bloomington, who is not part of Dr. Falk’s research was involved. “Females compete in all sorts of contexts, but only some of them have anything to do with competing for mates.”
The white-necked Jacobin weighs as much as a nickel and a penny, and the males are as long as a roll of toilet paper. The birds are also hams, often fanning their tails and turning backwards to show off. Dr. Falk calls them “the jocks of the hummingbird world”.
But their different colors in the females are a tedious puzzle. During his studies, Dr. Falk on a work published in 1950, in which a melange of white-necked female Jacobin hummingbirds was described. Some were green, but others were so convincingly masculine that the original collectors underlined the ♀ symbol twice for emphasis next to a blue-headed female specimen.
To find out why the female Jacobins resemble the males, Dr. Falk 2015 to the city of Gamboa in Panama, one of the more accessible places for the hummingbirds.
After sexing 401 birds that visited bird feeders in the city and in a nearby forest, Dr. Falk found that around 28 percent of all females resembled blue-headed males. More specifically, all of the young females had the male’s showy blue plumage, while the adornment tapered to 20 percent of the adult females. So all the juveniles looked like males, but as the females grew most of them shed their skin a muted green.
The discovery of the male pups did not correspond to the idea of sexual selection.
“They wear this beautiful ornament when they are not taking care of partners,” said Dr. Falk.
Dr. Falk wanted to see how the Jacobins would react to the green birds and the flashy blue birds. He painted clay mounts in the style of the birds, but the birds were not moved – artistically or sexually. So he turned to groomed mounts and placed combinations of green females, blue males, and blue females on feeding grounds to see how passing hummingbirds would react.
If sexual selection were involved, Dr. Falk hypothesized that the male hummingbirds would prefer the showy male females. But the males had a clear sexual preference for green females. And the Jacobins, as well as other species of birds, more often directed their territorial aggression towards green females than blue females and males, regardless of the sex of the mount, although they do not know why.
These experiments were consistent with the researchers’ recordings of hunts for white-necked Jacobins in the wild, which found that green females were hunted more than ten times as often as their blue-headed relatives.
The blue-haired women clearly enjoyed more personal space, but were there other advantages? To test this, Dr. Falk investigated the feeding behavior of green females, blue-headed females and blue-headed males with implanted tracking tags. An analysis of 88,000 feeding visits over nine months showed that the blue-haired females visited feeding places more frequently and for longer than the green females.
Forget men; Food appears to be the ultimate driver for feminine ornamentation in white-necked Jacobin hummingbirds. The blue-headed females feed longer and are hunted less – a blessing for a bird that burns energy like no other.
“Hummingbirds live vigorously on the edge,” said Dr. Rosvall. “Even the smallest benefit in eating is a real benefit.”
How exactly do some females stick to their male coloring remains a mystery. Dr. Falk said he hoped to study the mechanism behind these plumage.
If you see a green Jacobin and a blue-headed Jacobin in the wild, they might appear as a mating. But it could also be two females who spend their day completely unaffected by everyone’s assumptions.