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September 7, 2022
Jacobin white-necked hummingbirds sport colorful blue and white plumage when juveniles. When they reach adulthood, males retain this dazzling pattern, while females develop a more “muffled” palette of green and white – at least most females do. Curiously, about 20% of females defy the norm and retain male-like plumage as adults.
“Why do some Jacobin females look like males? It’s a mystery made up of many parts,” said Jay Falk, a postdoctoral researcher at the University of Washington. “Is there an advantage? Is there a cost? Is it just an appearance, or do these females also act like males? »
Now these pieces are falling into place. In research published Sept. 7 in Proceedings of the Royal Society B, Falk and co-authors from UW, Cornell University and Columbia University report that adult female white-collar Jacobins with a Male-like plumage mimic male appearance – but not male behavior. In addition, their strength and body size do not resemble males, but other females with muted plumage.
The study shows that 1 in 5 adult females with male-like plumage engage in “deceptive mimicry”: they essentially try to pass themselves off as males, without acting like them. In the process, they receive quite a benefit. As Falk and colleagues reported in an article published last year in Current Biology, females with male-like plumage suffer less aggression from males than females with more typical attenuated plumage, and may trail longer at feeders.
Falk began this research as a graduate student at Cornell University and continued it as a postdoctoral fellow with co-author Alejandro Rico-Guevara, assistant professor of biology at UW and curator of ornithology at Burke Museum of Natural History & Culture at UW.
White-necked Jacobins are common in the tropical lowlands of the Americas. Males of this species, simply put, are bullies. They defend territories, chase rivals away from food sources, court females, and fight each other. This aggressive behavior is based on an underlying difference in body size and physiology: male Jacobins are larger and are better at combat flight than dull-colored females.
An unanswered question from Falk’s earlier study was whether females with male plumage also displayed male-like flight power or behavior. At a field site in Panama, he briefly captured male and female Jacobins with both types of plumage. He found that females – regardless of plumage – had essentially identical body and wing sizes, while males were slightly larger. Before releasing the birds, Falk also tested their “burst power” – or muscle capacity during flight – by seeing how high they could fly while lifting a string of small weighted beads. Females of both plumage types had identical burst power, while males could lift more on average.
Using data from radio-tagged birds in the wild, the team also found that more males foraged in a ‘territorial’ pattern – spending more time at fewer foraging sites. All females, regardless of plumage, showed the opposite pattern: foraging for shorter periods at sites distributed over a larger territory.
“Females with male-like plumage don’t seem to behave any differently than other females,” Falk said. “Rather, all the evidence points to females who look like males engaging in deceptive mimicry.”
Many examples of deceptive mimicry occur between species: a harmless species will mimic the coloration of a pest species as an anti-predator defense. In the Americas, for example, some species of non-venomous queen snakes have developed colored banding patterns that resemble venomous species in the same region, such as coral snakes. Research has shown that this deceptive mimicry decreases predation by queen snakes, which are not venomous. What Falk and his colleagues found in white-necked Jacobins appears to be an example of deceptive mimicry within a species.
Scientists have reported females with male-like plumage in other hummingbird species. If so, male mimicry within hummingbird species may be more common than currently known. Next year, Falk will move to the University of Colorado at Boulder to study the genetic differences between dumb and male plumaged females – and potentially identify how this deception evolved.
But gender differences are not everything.
“Even when I found average differences in female and male morphology, bursting power, or demeanor, I also found some gender overlap,” Falk said. “This indicates that sex is not the only important factor, and that variation among and between individuals plays an important role.”
Falk and Rico-Guevara are currently investigating the role of individual variation in these traits, regardless of gender.
Other co-authors of the study are Michael Webster of Cornell University and Dustin Rubinstein of Columbia University. The research was funded by the Smithsonian Tropical Research Institute, the National Science Foundation, Cornell University, the Walt Halperin Endowed Professorship at UW, the Washington Research Foundation, the Society for the Study of Evolution, and the American Society of Naturalists.
For more information, contact Falk at [email protected]
Tag(s): Alejandro Rico-Guevara • Burke Museum of Natural History and Culture • College of Arts and Sciences • Department of Biology • evolution