Female Hummingbirds Masquerade as Males to Avoid Harassment

One-fifth of female white-necked jacobins sport flashy male-like plumage, which may help them access more food

A male white-necked jacobin hummingbird perched on a branch. He has white underparts, greenish wings, a bright blue hood and long black bill.
Around 20 percent of female white-necked jacobins have evolved to share the vibrant plumage characteristic of males. sam may via Flickr under [CC BY 2.0]

Female hummingbirds are ditching their drab plumage to avoid bullying, according to new research. Some female white-necked jacobins, which are usually a greenish color with a gray-dappled chest, have evolved to share the flashy blue-and-white plumage typical of their male counterparts. Scientists suspect male-look-alikes are implementing this evolutionary trick to avoid social harassment from their peers.

Most hummingbird species aren’t afraid to pick a fight, and white-necked jacobins, which range from Mexico to Brazil, are no exception. The fast-moving creatures need to eat multiple times an hour and fiercely defend food sources by chasing and pecking other birds—sometimes to death. Males claim territories to have a monopoly on breeding, and typically show less aggression toward trespassing females.

“Hummingbirds live on the margins energetically. An ever-so-slight advantage in acquiring food is a real advantage,” says biologist Kimberly Rosvall of Indiana University, Bloomington, who was not involved in the study, to the New York Times’ Sabrina Imbler.

The study, published last month in Current Biology, found that around one-fifth of female white-necked jacobins implemented this disguise trick—and did so almost indistinguishably from males. Because all chicks of this species begin life looking like males, scientists previously hadn't noticed the change.

A female white-necked jacobin perched on a branch. She has green plumage on her head, back and wings, and spotted grey color on her breast.
Unlike the bright blue and white coloration of males, female white-necked jacobins typically have more subdued hues. Alejandro Bayer Tamayo via Wikimedia Commons under [CC BY-SA 2.0]a>

“Every female and male start out looking like the adult males. Then as they age, about 20 percent of the females keep that plumage, and then 80 percent shift out into the drab plumage,” says study co-author Jay Falk, a pre-doctoral researcher at the Smithsonian Tropical Research Institute and Cornell University, to Natalie Grover for the Guardian. “So, it was clear something was at play.”

To see how plumage factored into harassment, researchers placed taxidermied mounts of the hummingbirds in Gamboa, Panama, and watched how the other hummingbirds interacted with the decoys. Compared with drab females, vibrant females with male coloration faced less social harassment from both male and female birds.

This plumage-swap phenomenon "is pretty unusual in birds, and it was so unexpected that it actually took me a few years to see it in the data," Falk tells Megan Marples for CNN.

The benefits of male plumage might seem obvious, but being overly ornamental can be risky, reports Katherine J. Wu for the Atlantic. Females with typical male coloring could be easier for predators to spot, for example. Food might be the reason female hummingbirds are willing to make that trade-off.

"The data suggests that these more aggressive females with the male-like plumage are better at defending a key food resource," Rosvall tells CNN’s Megan Marples. "They do more chasing and are chased less."

Nine months of observations showed that brightly-colored females were able to visit feeders more frequently and for longer than those with muted hues. The plumage change isn’t without drawbacks—the team found that a male-like appearance decreased a female’s chance of breeding success.

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