Why Do Some Female Hummingbirds Crossdress?

Some female hummingbirds mimic the plumage colors and patterns of males to avoid harassment from aggressive males without becoming bullies themselves

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A lot of people are familiar with the idea that many birds are color-coded by sex: males are brilliant whilst females are drab. But hummingbirds have turned this perception onto its head because in some species, a portion of adult females have flashy male-like plumage coloring and patterns that makes them visually indistinguishable from males. Why?

When males and females share chick-rearing duties equally, they typically have identical plumage colors and patterns because both males and females are choosing their mates. But plumage transvestism, where some females in a population are indistinguishable males, is especially common amongst hummingbirds. Yet paradoxically, only female hummingbirds build nests and raise chicks, so clearly something strange is happening with them.

“One of my advisors told me about this phenomenon and I thought it was so cool”, lead author of a recent study, evolutionary biologist and ornithologist Jay Falk told me in email.

“It’s definitely unexpected, and I think there’s real value in studying things that don’t fit our expectations in nature”, Dr Falk said. “I was surprised that no one had studied this outside of museum specimens, so I decided to take a crack at it.”

Whilst a graduate student at Cornell University, Dr Falk began studying this crossdressing phenomenon by examining museum specimens and found that it evolved independently in all nine major hummingbird groups. But this study did not reveal the evolutionary reason for male-like plumage in female hummingbirds, nor why it seems to be more common in hummingbirds than in other birds.

But drawing on his knowledge of hummingbird physiology, Dr Falk had an idea: he proposed that male-like plumage in female hummingbirds may be a way for them to avoid male aggression whilst feeding. To test this hypothesis, Dr Falk, who is now a postdoctoral researcher at the University of Washington and at the Smithsonian Tropical Research Institute (STRI), spent four years in Panama studying the white-necked jacobin, Florisuga mellivora, in the field.

The white-necked jacobin is a relatively common medium-sized hummingbird that ranges widely throughout Central and northern South America. Like all hummingbirds, the white-necked jacobin feeds on nectar, often from flowers in the canopy of tall trees but also from the flowers of a variety of stubbier shrubs and plants. Although several individual hummingbirds feed at the same tree, males aggressively chase away rivals from nectar patches, fight with each other for possession of these nectar patches, and court females — all of which are metabolically expensive activities.

Like all hummingbirds, white-necked jacobins are beautiful. Adult males have gleaming iridescent cobalt blue heads and upperparts, iridescent greenish-blue wings, that contrast sharply with snowy white underparts and tails, a long slender black beak, black eyes and a small white patch on the back of the neck from which they get their name. Adult female white-necked jacobins are drab in comparison — but only if you think an emerald is drab compared to a sapphire — with a lovely iridescent sea-green head, neck and upperparts, a tiny grey patch behind their black eyes, long slender black beak, wing and tail primaries, black speckles extending from the throat to the breast and a snowy white belly patch.

Despite these dramatic plumage differences between males and females, Dr Falk discovered that some adult female white-necked jacobins look identical to adult males.

“We knew based on a paper from 1950 that at least some of the females look like males (this was from museum specimens where they look at the gonads during the curation process)”, Dr Falk explained to me in email. “We didn’t know to what extent this was true, and whether it was throughout their wide range. So I did a pilot study [whilst a graduate student at Cornell] just to verify that they exist. I caught about 30 birds that year and took blood samples to test their sex — sure enough, there were a few in there that I thought were males based on color but turned out to be females.”

In that previously published pilot study, Dr Falk and his team reported that around 20% of adult females have flashy male-like plumage colors and patterns that make them indistinguishable from adult males (ref). Further, this isn’t a temporary phase: Dr Falk and his team’s banding data revealed that females with male-like plumage retain it throughout their lives.

Why did this male-like plumage color polymorphism arise? What evolutionary advantage does it provide?

Male-like plumage does not indicate these hummingbird females are some sort of Amazon warriors. For example, Dr Falk and his collaborators found that females, regardless of plumage type, had basically the same wing and body sizes, whereas males were slightly larger. Additionally, Dr Falk found that males had greater “burst power” — a measure of their muscle capacity during flight — than females of either plumage type, and that females of both plumage types had identical burst power. In short, females with male-like plumage do not maintain energetically expensive muscle mass.

Field studies also revealed behavioral differences: males spend most of their time at a small number of feeding sites within distinct territories. In contrast, all females, regardless of plumage type, feed for shorter periods of time across a much larger area. They also are less aggressive than males.

“We found that males were more likely than females to be territorial and have costly adaptations that facilitate this risky lifestyle”, Dr Falk explained.

Another clue to this plumage puzzle was provided by juveniles, because juveniles of both sexes resemble adult males.

“For birds, that’s really unusual because you usually find that when the males and females look different the juveniles usually look like the adult females, not the adult males, and that’s true almost across the board for birds”, Dr Falk said. “It was unusual to find a species where the juveniles looked like the males. So it was clear something was at play.”

That ‘something’ could be physiology: a hummingbird’s metabolic demands are intense — the highest amongst all vertebrates — and this may be the reason that juveniles and some adult females evolved a male-like plumage: to avoid most of the costs of male aggression and harassment whilst feeding.

“Competing hummingbirds avoid females that look like males, even though they are not as aggressive.”

Juveniles and adult females with male-like plumage can feed from flowers and hummingbird feeders longer without being bullied. For this reason, male-like plumage in hummingbirds appears to be an advantage because of a perceived association with aggression.

“Females with male-like plumage don’t seem to be behaving any differently than other females”, Dr Falk noted. “All evidence instead indicates that females that look like males are engaging in deceptive mimicry.”

Deceptive mimicry is an evolutionary adaptation to mislead or deter a potential aggressor of other species. But because access to food can be limited by individuals of the same species, looking like a male may deter social aggression from other white-necked jacobins. This suggests that Dr Falk and his collaborators may have identified the first known example of deceptive mimicry within a species.

What are the molecular mechanisms that gives rise to females with male-like plumage? Genetics? Transposons? Hormones? Or is there some other molecular mechanism that underlies male-like plumage? Are female offspring of ‘transvestite’ mothers more likely to grow up to be crossdressers themselves?

“Awesome question, I wish I knew!” Dr Falk replied to me in email. “This will be the full focus of my next postdoc starting in January, which will be funded by the NSF postdoctoral program. So I’ll be diving head-on into the these mechanistic and genetic questions.”

Source:

Jay J. Falk, Dustin R. Rubenstein, Alejandro Rico-Guevara and Michael S. Webster (2022). Intersexual social dominance mimicry drives female hummingbird polymorphism, Proceedings of the Royal Society B: Biological Sciences 20220332 | doi:10.1098/rspb.2022.0332

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