A newly published study found that the range of colors in the plumages of hummingbirds exceeds the total color diversity of all other bird species combined
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F I G U R E 1 | Diversity of hummingbird plumage color. Photographs of males of eight of the 114 … [+]
I’ve written about bird colors before (more here, here, here, here and here.) And, as you’ve probably noticed, there are a lot of extremely colorful birds in the world, especially in the tropics (for example, more here), but which group of birds are the most colorful of them all? And how do they create their rainbow of plumage colors? These were the questions investigated by a team of scientists at Yale University (some members of this team have since migrated to new locations), under the guidance of evolutionary ornithologist Richard Prum, who is the W. R. Coe Professor of Ornithology and the Curator of Ornithology at the Yale Peabody Museum. The findings indicated that the range of colors in the plumage of hummingbirds exceeds the total color diversity of all other bird species — combined. But how did the scientists come to this conclusion?
How hummingbirds create some of their colors
“I feel there is no way to avoid thinking of color when you think of hummingbirds”, said Gabriella Venable, who is the lead author of the newly published study of hummingbird color (ref). “We give them names like ‘mountain gems’, ‘brilliants’, ‘sapphires’, and ‘jewelfronts’. Their colorfulness was one of the things that attracted me to them as a child.”
“I carried this passion (and what I hope will be a lifelong love) for these tiny birds to undergrad”, Ms Venable said in email before adding: “I should note, however, that I no longer study hummingbirds.”
Ms Venable, who currently is pursuing her PhD in how cognition evolves at Duke University, was an ecology and evolutionary biology undergraduate at Yale University when she completed her hummingbird study.
“For my senior thesis, I knew I wanted to study hummingbirds, so I asked for advice from my advisor, Dr. Richard Prum, on what would be an interesting topic to study about these birds”, Ms Venable told me in email. “He recommended that I study hummingbird plumage color diversity because of their diverse barbule structural coloration” [ref].
Structural colors are produced by the microscopic structure of feather barbules themselves (Figure 2). Instead of pigments, structural colors are produced by special proteins in the feather that reflect one particular color. Blue is one such structural plumage color because blue pigments are very rare in nature, and are very expensive to manufacture. Bluebirds and jays rely on structural colors to create their blue plumages.
F I G U R E 2 | Anatomy of a feather. The parts of a feather you can see with the unaided eye are … [+]
But probably the most familiar example of structural color is iridescence. It is created by specialized microscopic structures inside feathers that split (refract) white sunlight into its separate spectral wavelenths, depending on the angle from which the feathers are viewed. These feathers create a rainbow of ephemeral colors in the same way that a crystal prism does, or a thin layer of oil sitting on the surface of water. These feather structures rely upon microscopic stacked layers of pancake-shaped melanosomes (special structures containing black or brown pigments) with tiny air pockets interspersed throughout. The air pockets scatter light and the melanin pigments absorb a specific range of spectral light colors, which causes the feathers to reflect a single, very intense, color. But iridescent feathers only create colors when light impinges upon the feather at a certain angle. If the feather is tipped away from that angle, the feathers appear black or brown due to their melanin pigments.
In birds, iridescence is typically seen in pheasants, ducks, pigeons, kingfishers, a variety of songbirds, such as starlings and birds of paradise, some lories (ref) — and especially in those champions of iridescence, the hummingbirds. Many hummingbird species have iridescent gorget (throat) feathers whilst other species are cloaked completely in a brilliant shimmering rainbow of eye-watering colors.
How do hummingbirds use colors?
Under the guidance of Professor Prum, Ms Venable measured more than 5000 reflectance spectra from over 1600 plumage patches located on 114 different species of hummingbirds (Trochilidae) from study skin specimens held by the Yale Peabody Museum and the American Museum of Natural History. This study sampled approximately 33% of all living hummingbird species (and 60% of all hummingbird genera).
Reflectance spectra were measured from six standardized patches on all specimens: crown, back, tail, wing, belly, and throat. Additional patches were measured if they looked distinct to the human eye and were large enough to measure reliably. These hummingbird color measurements were then compared to a previously published dataset of colors measured in 965 plumage patches of 111 other bird species, including penguins, finches, parrots, quetzals and, yes, more hummingbirds.
But hummingbirds do not create their rainbow of plumage colors to appeal to human eyes. Instead, they create these plumage colors to communicate with each other; to attract mates of the appropriate species, and to act as a brilliant flashing beacon to mark their territories.
“They use them in their social and sexual communication”, Professor Prum told me in email. “Achieved color diversity is a product of color production mechanisms and selection on them. Hummingbirds have a combination of extraordinarily versatile structural coloration in their feather barbules, and a long history of female mate choice (including, exclusive female parental care and polygyny). Hummingbird colors also function in competitive social interactions at flowers.”
Because plumage color varies as the feathers’ angle with the sun changes, the aerial movements of hummingbirds may be used to send messages to other hummingbirds in the area.
How do hummingbirds see colors?
This raises the question: how do hummingbirds themselves perceive these colors? To answer this question, Ms Venable, Professor Prum and their collaborators then compared the measured color spectra to what is known of avian visual physiology.
“Birds have four types of color cones that are sensitive to red, green, blue, and ultraviolet/violet colors”, Professor Prum told me in email. “So birds can see all the colors visible to humans. But birds can also see a host of other colors that humans cannot see, including ultraviolet, and mixtures of ultraviolet with other hues, like ultraviolet-yellow and ultra-violet green”, Professor Prum continued in email. “These colors are as different from yellow or green as purple is from blue or red.”
Remarkably, when Ms Venable, Professor Prum and their collaborators combined the total numbers of colors that they identified in hummingbird plumage with those colors in the database that had previously been identified in the larger sample of birds, the total variety of colors in the plumage of all bird species was increased by 56%. Further, they discovered that 84.5% of the total variety of colors previously identified in the plumages of all bird species are also found in hummingbird plumages.
“We knew that hummingbirds were colorful, but we never imagined that they would rival all the rest of the birds combined”, Professor Prum said.
Amongst the range of colors that hummingbirds can see, this study identified just 34.2%. These include those colors that are really difficult to create, like saturated blues, blue-greens and true purples. The patches of feathers with the greatest diversity of colors were the top of the birds’ heads and their throats — both areas are used for social signalling as well as to bewitch potential mates.
This astounding range of colors is due to the refractive nanostructures in hummingbird feather barbules.
“Watching a single hummingbird is pretty extraordinary”, Professor Prum said. “But the combination of versatile optical structures and complex sexual displays make hummingbirds the most colorful bird family of all.”
Favorite hummingbird species?
Okay, I had to ask the authors an obvious question: which is your favorite hummingbird species, and why?
“Oof. Hard question. My favorite species of hummingbird depends on the day”, Ms Venable told me in email. “I have several favorites because they remind me of my home or previous field work as well as favorites because of how beautiful they are.”
Adult male Emerald-bellied Puffleg (Eriocnemis aline dybowskii) Owlet Lodge, Abra Patricia, … [+]
“One of my favorites that I like to talk about though is the Emerald-bellied Puffleg”, Ms Venable elaborated in email. “This species, as well as other pufflegs, have very fluffy boot-like feathers around their feet, which are presumed to exist because of sexual selection. In other words, female pufflegs might like fluffy feet and that makes me happy.”
The pufflegs all have teensy fluffy dandelion-like hummingbird socks! What’s not to like about that?
Adult Violet-purple Coronet (Boissonneaua jardini) at the Sachatamia Lodge near Mindo, Ecuador. This … [+]
“Hard choice!!! So many delights!” Professor Prum replied after I’d posed the same question to him in email. “But the Violet-purple Coronet (Boissonneaua jardini) has got to be on the list. A narrow endemic to the west slope of the Andes of northern Ecuador and southern Colombia has amazing green, violet, purple and black colors. For years, I have jokingly called it the ‘Violent-purple Coronet.’ In our study, it turned out to be the most color-diverse plumage known among all birds!”
Personally, if I had to choose just one hummingbird species to gaze upon until the end of my days, I’d probably choose the violet-purple coronet too, because their incredible range of green-blue-violet-purple-black colors are at the end of the color spectrum that most appeals to me, also because males and females look alike and both are indistinguishably brilliant to human eyes.
Museum collections made this study possible
Did the authors do any field work to collect these color spectra data?
“This was purely a museum specimens-based study”, Ms Venable replied in email. “This work would absolutely not have been possible without the Yale Peabody Museum or the American Museum of Natural History”, she added in email.
“I was a tour guide for one of the research collections at the Yale Peabody Museum and the way I talked about the importance of museums was by explaining that they are like libraries. When writing a paper about history or literature, you might check out a book from a library”, Ms Venable elaborated in email. “Museum collections allow us to ‘check out’ study specimens, like the hummingbirds I studied.”
But museum collections serve the academic and public communities in so many other ways as well, from revealing the biodiversity of life to the public as well as educating students, to providing critically important samples for research and access to rare specimens for artists.
“Museums are also adapting in some ways to better fit modern science”, Ms Venable continued in email. “Many are becoming like online databases by uploading images of their specimens online, such that researchers, students, teachers, etc. can access them from anywhere. Additionally, many have begun to preserve tissue samples from specimens for the possibility of future molecular work.”
The overall importance of museum collections can be seen in some of the many important museum-based discoveries.
“The best example of the importance of museums is probably the discovery in the mid-1900s that DDT was killing birds by thinning their egg shells”, Ms Venable pointed out in email. “The majority of the specimens analyzed came from museum collections.”
“[T]his paper is entirely museum-based”, Professor Prum explained in email. “Measurements of these colors is so tricky that this study could not possibly be done with wild birds or even plucked feathers.”
What role do museum collections play in your research, Professor Prum?
“So much of my work is museum-based that it’s hard for me to think what it would mean to do research without them!” Professor Prum replied in email. “But museums make a critical and ongoing contribution to our understanding of the materiality and diversity of life.”
“Biology is not just about DNA and behavior!”
Source:
Gabriela X. Venable, Kaija Gahm & Richard O. Prum (2022). Hummingbird plumage color diversity exceeds the known gamut of all other birds, Communications Biology 5:576 | doi:10.1038/s42003-022-03518-2
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