New research on the glittering White-necked Jacobin hummingbird reveals
nearly 20% of the species’ adult females have male-like plumage. This
strategy is all about dodging bullies and getting better access to food. The
findings were published today in the journal Current Biology.
"What's interesting about the White-necked Jacobin is that all the juveniles
start out with male-like plumage," explains lead author Jay Falk. "Among
most other bird species, juvenile plumage looks more like the female's,
presumably to be less obvious to predators." Falk did this work while a
Ph.D. student with the Cornell Lab of Ornithology and is currently at the
University of Washington.
As the birds mature, all the jacobin males retain the fancier plumage but so
do nearly 20% of the females among the population Falk studied in Panama.
The remaining 80% of females still develop the muted green and white
coloration of a typical adult female. Though plumage ornamentation is
usually attributed to sexual selection and attracting a mate, researchers
ruled out that explanation for this species after field experiments.
Scientists observed the reactions of live jacobin hummingbirds toward
stuffed mounts placed on nectar feeders during breeding season.
The mounts were stuffed specimens adult White-necked Jacobin males, typical
adult females, and female adults that looked like males.
"If females having male-like plumage is the result of sexual selection, then
the males would have been drawn to the male-plumaged females," says Falk.
"That didn’t happen. The male White-necked Jacobins still showed a clear
preference for the typically plumed adult females."
So, what's the benefit to females of this species when they look like
a male? To get to the root of that puzzle, Falk and his assistants put radio
frequency ID tags on birds and set up a circuit of 28 feeders wired to read
the tags. By tracking the number and length of visits, they honed in the
answer."Our tests found that the typical, less colorful females were
harassed much more than females with male-like
plumage," Falk says. "Because the male-plumaged females experienced less
aggression, they were able to feed more often—a clear advantage."The
researchers found that the male-like females got to feed longer than the
typical adult female—about 35% longer at feeders filled with high-sugar
nectar.
That can make a big difference because hummingbirds have the highest
metabolic rate of any vertebrate. They need to eat constantly in order to
survive.
Bottom line: female White-necked Jacobins retain the male-like plumage of
their youth for social reasons—they avoid the bullies by looking like them.
It is still not clear whether male-like females behave just as aggressively
as the males. The actual physical mechanism that allows females to retain
male-like plumage is also not known.
The White-necked Jacobin is hardly alone when it comes to having some
females that look like males. Falk says studies have found that 25% of the
world’s 350+ hummingbird species also have some females that look like
males.
Reference:
Falk JJ, Webster MS, Rubenstein DR. Male-like ornamentation in female
hummingbirds results from social harassment rather than sexual selection.
Current Biology. 2021;0(0).
doi:10.1016/j.cub.2021.07.043