Some Female Hummingbirds Look Like Males to Avoid Bullying

In many species, males are so much flashier than the females. It’s one example of sexual dimorphism, where there are obvious differences in the size or appearance between genders. 

Being bigger and bolder-looking helps males compete against their rivals and win mates. In at least one hummingbird species, females have figured out that it’s a good advantage, so they look like males in order to evade aggression and get more food.

A new study finds that with the white-necked Jacobin hummingbird, almost 20% of adult females have male-like showy plumage. But it’s even more common when they are young.

“In bird species where the females and males look different, we almost always see that the juveniles tend to look more like the adult females. However, in this species we found that all juveniles tend to look like the adult male,” study lead author Jay Falk tells Treehugger. “From our research in this project, we suspect that it has something to do with escaping aggression from other hummingbirds.”

Falk studied the bird in Panama while a Ph.D. student with the Cornell Lab of Ornithology and is now a postdoctoral researcher at the University of Washington.

Falk and his colleagues found that as the hummingbirds matured, all the males kept their elaborate plumage, but so did 20% of the females. The rest of the females developed the muted green and white colors that were typical for adult Jacobin females.

Researchers were curious how the females benefited when they looked like males, so they set up experiments to find out. They attached radio frequency ID tags on hummingbirds, then set up nectar feeders around the town of Gamboa, Panama, during breeding season. The feeders were equipped to detect and read the tags. Then the researchers placed stuffed mounts on each feeder of either a male, typical female, or male-like female Jacobin.

“We then simply watched how other hummingbirds interacted with those mounts as they approached to feed,” Falk says. “Overall, we were able to find that male-like female and male mounts received less aggression from other hummingbirds than the drab females.” 

Because the females with male plumage were harassed less, they were able to feed more often, which was an obvious advantage, the researchers found. The ability to have more access to food is likely key to hummingbirds because their metabolism is so high.

The researchers discovered that male-like female hummingbirds were able to feed about 35% longer than typical adult females. That can be a big advantage because hummingbirds have the highest metabolic rate of any vertebrate. They must eat constantly to survive.

The results were published in the journal Current Biology.

Other Clever Birds

While the female white-necked Jacobins are able to avoid bullies by looking like males, researchers aren’t sure if they pick up any male traits. “In our analyses so far, we're not sure whether male-like females are equally as aggressive as males,” Falk says.

The white-necked Jacobin isn’t the only clever species where some females use the advantages that come with looking like males. Falk says studies have found that 25% of the more than 350 species of hummingbirds in the world have some females that look like their male counterparts.

He adds, “But it's not always to the degree we see in white-necked Jacobins where the male-like females are nearly indistinguishable from the males.”