Fig. 2. Simplified external anatomy of a typical bird (non-passerine green heron): 1 Beak, 2 Crown feathers, 3 Iris, 4 Pupil, 5 Nape, 6 Neck and Throat, 7 Auricular, 8 Malar, 9 Breast, 10 Scapulars, 11 Coverts, 12 Belly, 13 Primaries, 14 Tertials, 15 Secondaries, 16 Vent, 17 Tibia, 18 Tarsus, 19 Toe. (Photo: Scott Young)
Fig.3. Includes: 1 Median crown strip, 2 Lateral crown stripe, 3 Supraloral area, 4 Lore, 5 Supercillium or eyebrow stripe, 6, Eyeline or eyestripe, 7 Auricular or cheek, 8 Mustache or moustachial stripe, 9 Upper mandible, 10 Lower mandible, 11 Throat, 12 Malar stripe, 13 Submoustachial area, 14 Eye arc or broken eyering, 15 Nape, 16 Mantle, 17 Scapulars, 18 Breast. (Photo: Scott Young)
Have you wondered how birds can be so colorful, what gives them their coloration? Basically, it comes down to essentially six known processes, although each is quite intricate and there is still a lot to learn about them.
The six processes are: syntheses and depositions of 1) eumelanins, 2) pheomelanin 3) psittacofulvins, and 4) porphyrins; 5) ingestion and processing of pigment precursors (for example, carotenoids and vitamin A); and 6) structural modifications of the feathers.
- Eumelanins lead to brown and black colors. They are formed by complicated enzymatic processes that begin with the amino acid tyrosine. This process, melanogenesis, leads to the polymer pigments being transported in the melanophores of melanocytes to the feathers where the melanophores are taken up. The more oxidation, the darker the eumelanin. Also, eumelanin deposition helps strengthen the feathers and are essential for complex plumage patterns (Galván et al., 2017). The “brown” mutation in which normally black feathers become brown, results from decreased oxidation.
- Pheomelanins lead to yellow to reddish colors and share some of the synthetic pathway used for eumelanin synthesis. Pheomelanins specifically incorporate cysteine derivatives into the pigment polymer.
- Psittacofulvins are especially prominent in parrots (including budgerigars) and lead to their bright-red, orange, and yellow colors. There seems to still be a bit of uncertainty as to the biosynthesis of these pigments, but most evidence supports an endogenous synthesis from fatty acids (Cooke et al., 2017) that is not dependent on ingested carotenoids.
- Porphyrins are synthesized within the bird and deposited in the feathers as a bright red under ultraviolet light. Under visible light, they can display a range of colors from pale pink to stunning greens and reds. Recently, the Great Bustard (Otis tarda) was shown to deposit certain porphyrins as a pink color in feathers of their bellies. The pink color is readily photo-bleachable and may present a transient reproductive signal (Galván et al., 2016). Turacoverdin and turacin are porphyrin-derived pigments responsible or bright green and red colors, respectively, of turacos (family Musophagidae) in Africa. Turacoverdin is the only known green pigment in birds. Green in other birds is produced though a combination of yellow pigments (from carotenoids or psittacofulvins, for example) and the blue color from structural feather features (see below).
- Ingestion of carotenoids leads to deposition of yellow, orange and red pigments. Carotenoids are produced by many organisms, including plants and crustaceans. The carotenoids may or may not be modified. There is evidence to suggest that intensity of pigmentation is correlated with fitness (Weaver et al., 2018). A great example of carotenoid coloration is the American Flamingo (Phoenicopterus ruber), a bird that would be much paler without its diet of crustaceans and plant plankton.
About 10% of starling species have bright coloration due to carotenoid ingestion. Interestingly, the strong yellow plumage of the Golden-breasted Starling (Cosmopsarus regius) does not use carotenoids directly for this color. Instead, for the yellow this African bird deposits vitamin A, a smaller retinoid derived from carotenoid, (Galván et al., 2019), a unique situation at this point.
- Finally, fine structural modifications of feathers can lead to blue and green colors as well as iridescence (as seen in Eurasian Starlings (Sturnus vulgaris)). Although some of the general principles (for example, wave interference) have been understood to some degree for over 300 years, much progress has been made recently regarding the microscopic structural requirements. These include thin-film microstructures and nanochannel matrices that reflect particular wavelengths of light. Lawe’s parotia (Parotia lawesii) bird-of-paradise uses structural elements to produce both green and blue colors for example.
Recently, a different form of structural color approach was found in paleognaths such as the Southern Cassowary (Casuarius casuarius). This coloration, dubbed “structural gloss,” depends on their narrow feathers having more exposed rachis (central shafts). This allows greater directional reflectance (specular reflectance), as from a mirror (Eliason & Clarke, 2020). The interested reader is referred to further literature (van Grouw, 2019; Price-Waldman & Stoddard, 2021).
Mutations in color formation add to the variety in bird coloration as well. Albinism, due to the genetic loss of tyrosinase necessary for formation of the melanins, results in white feathers and pink eyes and legs. Feathers that are colored by non-melanins (for example, carotenoids) are unaffected. This is actually an uncommon mutation in birds, in part because they become less fit due to weaker feathers. As this is an inherited defect in all cells, there is no such thing as partial albinism in birds.
Another mutation that leads to white feathers is leucism, due to the failure of melanocytes carrying the pigments to the feathers and skin to either develop or migrate. Again, non-melanin-colored feathers are unaffected. This discoloration is generally symmetrical, present at and unchanged from birth, and may be complete or partial. Progressive graying is a much more common cause of white feathers and often mischaracterized as partial leucism. This defect is characterized by progression with aging and asymmetric appearance of white feathers. There are a number of less common mutations and the interested reader is directed to the review by van Gouw (2018).
Finally, color can be added cosmetically by a bird’s secretions or other actions (Delhey et al., 2007). The yellow bill colors of many hornbills are produced by secretions from the uropygial gland, for example. Amazingly, already pink American Flamingos add further red color by smearing oil from its uropygial gland as well (Amat et al., 2011)! Other birds, such as some vulture species add color from the soil or mineral rich water. (Negro et al., 1999; van Overveld et al., 2017).
Amat, J. A., Rendón, M. A., Garrido-Fernández, J., Garrido, A., Rendón-Martos, M., & Pérez-Gálvez, A. (2011). Greater flamingos Phoenicopterus roseus use uropygial secretions as make-up. Behavioral Ecology and Sociobiology, 65(4), 665-673. https://doi.org/10.1007/s00265-010-1068-z
Cooke, T. F., Fischer, C. R., Wu, P., Jiang, T. X., Xie, K. T., Kuo, J., Doctorov, E., Zehnder, A., Khosla, C., Chuong, C. M., & Bustamante, C. D. (2017). Genetic Mapping and Biochemical Basis of Yellow Feather Pigmentation in Budgerigars. Cell, 171(2), 427-439.e21. https://doi.org/10.1016/j.cell.2017.08.016
Delhey, K., Peters, A., & Kempenaers, B. (2007). Cosmetic coloration in birds: occurrence, function, and evolution. Am Nat, 169 Suppl 1, S145-58. https://doi.org/10.1086/510095
Eliason, C. M., & Clarke, J. A. (2020). Cassowary gloss and a novel form of structural color in birds. Sci Adv, 6(20), eaba0187. https://doi.org/10.1126/sciadv.aba0187
Galván, I., Camarero, P. R., Mateo, R., & Negro, J. J. (2016). Porphyrins produce uniquely ephemeral animal colouration: a possible signal of virginity. Sci Rep, 6, 39210. https://doi.org/10.1038/srep39210
Galván, I., García-Campa, J., & Negro, J. J. (2017). Complex Plumage Patterns Can Be Produced Only with the Contribution of Melanins. Physiol Biochem Zool, 90(5), 600-604. https://doi.org/10.1086/693962
Galván, I., Murtada, K., Jorge, A., Ríos, Á., & Zougagh, M. (2019). Unique evolution of vitamin A as an external pigment in tropical starlings. J Exp Biol, 222(Pt 11), jeb205229. https://doi.org/10.1242/jeb.205229
Negro, Margalida, Hiraldo, & Heredia. (1999). The function of the cosmetic coloration of bearded vultures: when art imitates life. Anim Behav, 58(5), F14-F17. https://doi.org/10.1006/anbe.1999.1251
Price-Waldman, R., & Stoddard, M. C. (2021). Avian Coloration Genetics: Recent Advances and Emerging Questions. J Hered, esab015. https://doi.org/10.1093/jhered/esab015
van Grouw, H. (2018). White feathers in black birds. British Birds, 111, 250-263.
van Grouw, H. (2019). Plumage, Structure, and Colour. BTO NEWS, 19, 16-19.
van Overveld, T., de la Riva, M., & Donázar, J. A. (2017). Cosmetic coloration in Egyptian vultures: Mud bathing as a tool for social communication. Ecology, 98(8), 2216-2218. https://doi.org/10.1002/ecy.1840
Weaver, R. J., Santos, E. S. A., Tucker, A. M., Wilson, A. E., & Hill, G. E. (2018). Carotenoid metabolism strengthens the link between feather coloration and individual quality. Nat Commun, 9(1), 73. https://doi.org/10.1038/s41467-017-02649-z