Female hummingbirds have developed a trick to keep males away

Researchers report that some female white-necked Jacobin hummingbirds have male-like plumage and mimic the appearance of the male — but not male behaviour.

In addition, their strength and body size are the same not with males, but with females with faint plumage.

The white-necked Jacobin hummingbird features juvenile blue-and-white colored plumage. As they grow into adulthood, males retain this dazzling pattern, while females develop a more “mute” green and white color palette—at least most females.

Oddly enough, about 20% of females defy the norm and keep the males feathers in adulthood.

“Why do some female Jacobins look like males? It’s a several-piece puzzle,” says Jay Falk, a postdoctoral researcher at the University of Washington. “Is there a benefit? Is there a cost? Is it just appearance, or do these females act like males too? “

With the results of a new study, these puzzle pieces are in place.

These male hummingbirds are big bullies

The study shows that 1 in 5 adult females with male-like feathers participate indeceptive imitation. They are basically trying to portray themselves as male, without acting like themselves.

In the process they get a great benefit. As Falk and his colleagues reported in a paper published last year in current biologyFemales with male-like plumage are less aggressive than males than females with more common silent plumage, and can spend more time at feeders.

Falk began this research as a graduate student at Cornell University and followed it up as a postdoctoral fellow with co-author Alejandro Rico Guevara, an assistant professor of biology and curator of ornithology at the Burke Museum of Natural History and Culture.

The white-necked jacobine is common in the tropical lowlands of the Americas. Simply put, males are of this type bullies. They defend territory, chase competitors away from food sources, court females, and fight. This aggressive behavior depends on a fundamental difference in body size and physiology: male Jacobins are larger and better at combat flight than paler-coloured females.

One of the unanswered questions from Falk’s previous study was whether females with male plumage also exhibit male-like strength or behavior. At a field site in Panama, he briefly captured male and female Jacobin with both types of plumage.

He discovered that females – regardless of plumage – had identical body and wings sizes, while males were slightly larger. Before releasing the birds, Falk also tested “burst force” – or muscle amplitude in flight – by seeing how high they could fly while lifting a string of small, weighted beads. Females of both types of feathers had identical blast strength, while males could lift more on average.

Using data from radio-tagged birds in the wild, the team also discovered that more males were feeding in a “territorial” pattern — spending longer periods of time at fewer feeding sites. All females, regardless of plumage, showed the opposite pattern: feeding for shorter periods of time at sites across a larger area.

Females with male-like feathers do not appear to behave any differently than other females, Falk says. “All evidence instead suggests that females who resemble males engage in misleading mimicry.”

Other species also use deceptive imitations

Many examples of deceptive mimicry occur between species: harmless species will mimic the coloration of harmful species as a defense against predators.

In the Americas, for example, some non-venomous king viper species have developed colorful band patterns that resemble venomous species in the same region, such as coral snakes. Research has shown that this deceptive tradition reduced predation by king snakes, which are not venomous. What Falk and colleagues found in the white-necked jacobine appears to be an example of deceptive mimicry within the species.

Scientists have reported that females have plumage similar to males in other hummingbird species. If so, male mimicry within hummingbird species may be more common than is currently known. Next year, Falk will move to the University of Colorado Boulder to study the genetic differences between females with muted and male-like plumage—and possibly determine how this illusion evolved.

But the differences between the sexes are not the whole story.

Even when intermediate differences were found in male and female morphology, explosion forceor behavior, also found quite a bit of overlap between the sexes,” Falk says. “This indicates that gender is not the only important factor, and that inter-individual variation plays an important role.”

Falk and Rico Guevara are currently studying the role of individual difference in these traits, regardless of gender.

The study appears in Proceedings of the Royal Society B. Additional co-authors are from Cornell University, Columbia University, and the University of Washington.

The work was funded by the Smithsonian Tropical Research Institute, the National Science Foundation, Cornell University, Walt Halperin Foundation professorships at the University of Washington, the Washington Research Foundation, the Society for the Study of Evolution, and the American Society of Naturalists.

source: University of Washington