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Parrots use the same molecules to create magenta, red, and orange plumage colors, and almost identical molecules to make yellow plumage, but these molecules create different colors based on how they are physically arranged inside the feather structure
With rare exceptions, parrots are colorful -- many of them, brilliantly so. Although most birds with so-called “warm” plumage colors, like cardinals and goldfinches, for example, get their bright red, orange and yellow colors from pigments in their diet, parrots are different: they biochemically synthesize their color molecules, which are known as psittacofulvins. But when psittacofulvins are extracted from feathers and analyzed by curious scientists, they appear orange in solution. So where do parrots’ reds come from? According to a new study by a team of scientists in New Zealand, parrots combine the chemistry and physics of psittacofulvins to create a range of brilliant hues.
Parrots’ brilliant colors are created by psittacofulvins
Most birds color their plumage in several ways: first, they use structural colors, which are physics-based methods to create color. For example, violets and blues are created when light bounces off precisely-spaced microscopic layers within feathers (read more about structural colors: blue, violet and white plumages and about super-black plumages). In contrast, red, pink, orange and yellow in most birds are created by chemically distinct pigments, including carotenoids, that absorb particular wavelengths of light. These pigment molecules are deposited into the growing feather, thereby providing its color. Most birds get carotenoid molecules from plants or invertebrates in their diet, which helps explain why some zoo flocks of “pink flamingos” turned white, and why other bird species end up with aberrant plumage colors that make them look like hybrids (more here).
But amongst birds, parrots are special. Parrots do not depend on their diet for color pigments. Along with penguins, turacos and bustards, parrots are amongst a very few birds that we know of that biochemically manufacture their own colorful plumage pigments. In parrots, these molecules are chromophores known as “psittacofulvins” because parrots are the only animals that make them.
Like pigments, chromophores provide color, but chromophores differ from pigments.
“A chromophore is any part or configuration of a molecule that is responsible for producing colour through the absorption of light,” said study co-author, ornithologist and applied chemist, Daniel Thomas, who is a senior lecturer in zoology at Massey University of New Zealand. “‘Pigment’ and ‘dye’ are synonyms for molecules that absorb (typically visible) light.”
But are magenta, red, and orange hues in parrots’ plumage each produced by its own structurally distinct psittacofulvin? Maybe not. A recent study into the biochemical nature of psittacofulvins made the unexpected discovery that yellow and red feathers apparently contained very similar psittacofulvins (more here; ref). Additionally, observations of individual feathers show that psittacofulvin-colored feathers are unlike carotenoid-pigmented feathers because yellow and red colors can occur in the same feather, often associated with a color gradient, indicating that plumage coloration is manipulated during feather growth.
Another study found that when psittacofulvins were extracted from red feathers, they appear orange in solution (ref). Why? The authors of that study proposed the color shift that they observed may be the result of a conformational change in the molecule -- perhaps a straight molecule became bent in solution, or vice-versa, and that changed its light reflective properties, which then gave it a different color? Or alternatively, perhaps psittacofulvins create different color hues based on their molecular environment rather than solely due to their biochemical structure?
To shed some light on those questions, physical chemist Jonathan Barnsley, a PhD candidate at Dodd-Walls Centre for Photonic and Quantum Technologies in Otago, New Zealand, and his collaborators analyzed psittacofulvins inside individual parrot feathers to determine whether the in situ organization of psittacofulvins affects the feather’s hue.
Mr. Barnsley started by analyzing red and yellow areas on an individual tail feather kindly donated by a yellow-naped Amazon parrot, Amazona auropalliata.
Mr. Barnsley and his colleagues used Resonance Raman spectroscopy (RR spectroscopy), which takes advantage of the fact that molecules vibrate when illuminated by laser light. RR spectroscopy is especially sensitive to specific colored molecules that are present at very low (micro to millimolar) concentrations in an otherwise complex mixture of compounds.
RR spectroscopy involves shining a laser onto a sample and measuring the bond vibrations within molecules in the sample by an inelastic scattering process where an absorbed photon is reemitted at either a lower or higher energy. The difference in energy between the absorbed and reemitted photons corresponds to the energy of a molecular vibrational mode (i.e.; stretching of all carbons connected by double bonds), and the wavelength of reemitted photons is detected and used to study the structure of the molecule.
“Each molecule can have a number of different vibrations which make up a signature ‘chord’ and we detect that,” Mr. Barnsley said.
“This vibration information tells us what the molecules are and what they’re up to in the sample.”
After analyzing feathers from 25 species of parrots, Mr. Barnsley and his collaborators found that a parrot’s red feathers contain the same molecules used to make yellow feathers -- but they may be arranged differently, which means they interact differently with their neighboring molecules and with light, and this may be how different colors are produced.
“Chromophore diversity can come about when molecules interact or ‘communicate’ with their nearest neighbours to change how one another absorbs light,” Mr. Barnsley explained.
“Consider a group of light-absorbing molecules where some are aligned with close neighbours who change one-another’s shape, and some are not organised alongside neighbours so their shape is unchanged,” Dr. Thomas elaborated in email. “This group of molecules would contain a diversity of chromophores.”
“Bending a light-absorbing molecule into a different shape, which can occur when molecules interact with close neighbours, can cause that molecule to become a different chromophore (i.e change colour),” Mr. Barnsley pointed out.
Mr. Barnsley and his collaborators propose that parrots produce different mixtures of psittacofulvin chromophores during feather growth, and release these molecules into the growing feather at different times and at different concentrations to produce magenta, red, orange and yellow coloration and color patterns.
“This is a possible explanation we give for our observations of red feather colouration -- perhaps the base colour of these molecules is orange and through interactions they become red?” Dr. Thomas said in email. “Alternatively, chemically-different molecules that absorb light at different wavelengths might have little to no structural organisation and be well mixed in a sample. These ‘disorganised’ pigments might behave as a single chromophore from the perspective of our analyses, and this is a possible explanation we give for our observations of yellow feather colouration -- perhaps these yellow molecules don’t organise the same way as the orange ones and just stay yellow?”
Mr. Barnsley found that some parrot species’ feathers absorbed ultraviolet light, which is invisible to humans, and reemitted it as visible colored light -- a phenomenon known as fluorescence. He suspects this also results from molecular arrangements within the feathers.
Mr. Barnsley and his collaborators also found that the multiple chromophores in red feather barbs absorb over a range of wavelengths and could be the basis of a color-tuning mechanism, where major differences in color are associated with differences in the organization of psittacofulvins in the feather barb, whereas minor differences in hue may be linked to different chromophore concentrations. Further, “color-tuning” across a range of hues may account for major differences in plumage coloration between closely related species. Dr. Thomas speculated that it may be possible that parrots “tune” psittacofulvins to produce a whole range of colors -- from the magenta of a galah to the yellow of a sulphur-crested cockatoo.
Such color-tuning could be a significant factor in the evolution of parrots and maybe for other animals, too. For example, color-tuning has been identified in crustaceans, but has not previously been recorded in feathers.
“There is still important work to do in linking chromophore diversity to colour-tuning,” Dr. Thomas cautioned in email. “But it is exciting to think that we have a new way of exploring the long-standing question of why different animals generate similar colours using wildly different pigments.”
Source:
Jonathan E. Barnsley, Elliot J. Tay, Keith C. Gordon, and Daniel B. Thomas (2018). Frequency dispersion reveals chromophore diversity and colour-tuning mechanism in parrot feathers, Royal Society Open Science, 5:172010 | doi:10.1098/rsos.172010
Also cited:
Thomas F. Cooke, Curt R. Fischer, Ping Wu, Ting-Xin Jiang, Kathleen T. Xie, James Kuo, Elizabeth Doctorov, Ashley Zehnder, Chaitan Khosla, Cheng-Ming Chuong, and Carlos D. Bustamante (2017). Genetic Mapping and Biochemical Basis of Yellow Feather Pigmentation in Budgerigars, Cell, 171(2):427-439.e21 | doi:10.1016/j.cell.2017.08.016
R. Stradi, E. Pini, and G. Celentano (2001). The chemical structure of the pigments in Ara macao plumage, Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 130(1):57-63 | doi:10.1016/S1096-4959(01)00402-X
Read more about the science of animal color on Forbes:
GrrlScientist. “Fifty Shades of Black: These Bird Feathers Are The Darkest Never Seen”, Forbes, 11 January 2018. (link.)
GrrlScientist. “Scientists’ Colorful Quest To Discover How Parrots Became Green”, Forbes, 13 November 2017. (link.)
GrrlScientist. “Scientists Solve The Mystery Of Why These Yellow Woodpeckers Got Red Wings”, Forbes, 12 October 2016. (link.)
GrrlScientist. “Turtles Are Key To Tracing Birds’ ‘Redness Gene’ Back To The Dinosaurs”, Forbes, 3 August 2016. (link.)
GrrlScientist. “These Lizards Have Been Playing An Evolutionary Game Of Rock-Paper-Scissors For Millions Of Years”, Forbes, 15 June 2016. (link.)
GrrlScientist. “How Birds Became Red”, Forbes, 20 May 2016. (link.)
Parrots Use Chemistry And Physics To Create Brilliantly Colorful Plumage | @GrrlScientist
