When it comes to birds, sexual dimorphism can be very pronounced.
Males may announce their presence with showy feathers, while females of the same species are much more sedated, an adaptation believed to be related to sexual selection and reproduction.
Although not universal for all birds, it was thought to be the case of the white-necked Jacobin (Florisuga mellivora). But now scientists have discovered that a significant number of females of this species of hummingbird have bright colors, just like males.
This, they found, helps prevent aggressive behavior that males tend to target towards female hummingbirds during feeding, including woodpeckers and body blows to assert dominance over food.
“One of the‘ aha ’moments of this study was when I realized that all juvenile females had bright colors,” said ornithologist Jay Falk, now of the University of Washington, and formerly of the Cornell Lab of Ornithology and Smithsonian Tropical Research Institute.
“For birds it’s really unusual because it’s usually found that when males and females are different, young ones tend to look like adult females, not adult males, and that’s true almost generally for birds.
“It was unusual to find one where the young looked like the males. So it was clear that something was at stake.”
(William Stephens, iNaturalist, CC-BY and Erland Refling Nielsen, iNaturalist, CC BY NC)
The difference between the male and female plumage of the white-necked Jacobin is striking. Males (left, pictured above) have a bright blue head, green back and white belly and tail, with a white band on the back of the neck. Females (right) are duller green and black, with white neck and belly stained with darker feathers and darker tails.
Or rather, this is what we thought. As Falk and his team noted, all young people – male and female – have more colorful plumage.
Although they mature, only about 80% of women change their clothes. The remaining 20% maintained their showy doubts, which made them virtually indistinguishable from men at a glance.
The reason for this was not exactly clear, so the researchers designed an experiment to observe how birds interact based on their coloration. They installed a hummingbird feeder and added different taxidermy mounts to observe how this can alter the interactions of visiting wild birds.
There were three combinations of two mounts: heterochromatic male and female (of moderate coloration); male and androchromatic female (male-like colored plumage); and both heterochromatic and androchromatic feminine.
Hummingbirds tend to compete aggressively for food resources, even among their own species, so researchers recorded both sexual and aggressive interactions between birds.
They attempted to determine whether androchromatic females were preferred as pairs, suggesting that their bright coloration had a benefit for sexual selection.
Interestingly, males sexually preferred sharper colored females. In 100 percent of the trials, the first sexual advance was made toward a heterochromatic female, ruling out the selection of pairs as a reason for the bright coloration.
However, striking androchromatic females were less frequently the target of aggression during feeding when paired with a heterochromatic female. When both taxidermy mountains, a male and a female, had a vivid coloration, wild males showed no bias toward where they directed their aggression.
This suggests that dressing up as males reduces the speed with which females are socially harassed by males, which in some cases seems preferable to attracting them sexually. Raise your hand if you can relate.
“During the 26 trials of the mountain experiment, we witnessed 1,790 cases of aggression in the mountains .. and 359 chases with wild-necked white Jacobins,” the researchers wrote in their paper.
“Therefore, aggression was frequent and androchrome plumage effectively reduced these encounters.”
Interestingly, this could explain the unusual coloration of young people. As Falk noted, most species of sexually dimorphically feathered birds tend to have more camouflaged adult female coloration, which can help protect vulnerable creatures from predators. The bright coloration of the young suggests that, at least in white-necked Jacobins, they need greater protection from their own relatives.
The team hopes, in future research, to use their findings to find out how and why other species may have evolved into sexually dimorphic traits.
“Hummingbirds are animals so loved by many people, but there are still mysteries we haven’t noticed or studied,” Falk said.
“It’s great that you don’t have to go to an obscure unknown bird to find interesting and revealing results. You can only look at a bird that everyone likes to see in the first place.”
The research has been published in Current biology.