Flashy birds, from peacocks to parrots, have inspired a long tradition of using animal colors to study evolution. Colorful plumage often functions as a signal—for example, to attract a mate. Alternatively, drab colors can be used for camouflage. However, color can have functions beyond signaling and camouflage. This is because pigments, the molecules embedded in feather tissues to produce a broad range of colors, can also influence other aspects of feathers, ranging from mechanical strength to thermoregulatory properties. For example, melanin, a pigment that creates dark colors in many animals, has been shown to make feathers physically stronger. Melanin has also been shown to reduce the damage caused by bacteria that feed on keratin proteins, the building blocks of feathers. Beyond melanin, could the pigments responsible for vivid colors also have such non-signaling functions? New work led by Princeton University biologists sheds light on this question.
Parrots are hailed for their colorful plumage, yet psittacofulvins, the pigments primarily responsible for their brilliant colors, have remained somewhat of a mystery. Most birds use carotenoid pigments to produce bright red, yellow, orange and pink plumage. But not parrots. They evolved psittacofulvins, a group of pigments found in no other animals on the planet, to produce the same hues typically made by carotenoids in other birds. Why did parrots evolve a unique coloration system? A pioneering study in 2010 offered an answer, proposing that psittacofulvins might protect feathers from keratin-eating bacteria. Because feathers have so many functions, ranging from flight and thermoregulation to signaling and camouflage, susceptibility to bacteria-induced damage could have major effects on fitness. If psittacofulvins protected feathers from bacterial degradation, it might explain why these pigments—and their resulting colors—evolved in parrots. Using a relatively small sample of parrot feathers and the bacteria Bacillus licheniformis , which is ubiquitous in wild birds, the 2010 study found that red feathers, which have high concentrations of psittacofulvins, were resistant to bacterial attack, similar to black feathers.
In a new study published in the Journal of Experimental Biology , Ali et al. (2026) revisit the hypothesis that psittacofulvins protect parrots from microbial damage, but with a broader range of feathers from more species. The research team sampled thousands of parrot feathers, placing them in a solution inoculated with B. licheniformis . When these bacteria feed on feathers, small proteins are released into the surrounding liquid medium—making it possible to quantify the amount of feather breakdown that has occurred. By using the feathers of closely related parrot species, the researchers could control for variation in feather properties that would otherwise affect their findings. The least degraded feathers were black feathers, which was expected given their high melanin content. However, red, orange, yellow, and green feathers, all of which contain psittacofulvins, broke down the most, while feathers lacking psittacofulvins—blue, white, black and grey—fared substantially better. Given the results of the 2010 study, these results were unexpected.
“We thought psittacofulvins would combine with melanin to protect feathers,” explained lead author and former Princeton graduate student, Jarome R. Ali, “so our findings were entirely unanticipated”. Ali, now a postdoctoral researcher at New York University, continued, “The red feathers should have been intact, but they had totally fallen apart, while other colors, like blue, seemed mostly unbothered. We think our results may differ from those previously reported because we relied on a broader array of parrot feathers, which we sampled quite extensively. Parrots often seem to break the rules. In retrospect, we shouldn’t be surprised that they surprised us.”
Why do psittacofulvins provide such meek protection? The researchers suggest that incorporating psittacofulvins into the feather keratin could slightly destabilize its structure, making it susceptible to the enzymes bacteria use to break it down. Such a trade-off would have interesting evolutionary implications. “Our results suggest that the pressures favoring the evolution of the vibrant colors in parrots outweigh the potential negative effects of bacterial degradation,” said the senior author Mary Caswell Stoddard, professor of ecology and evolutionary biology at Princeton. “In other words, beauty over bacteria: parrots are often red, orange, yellow and green even though these colors are quite susceptible to bacterial attack.”
Alternatively, psittacofulvins might have some as-yet undiscovered role, in addition to producing beautiful colors. For example, their chemical structure is similar to some known antimicrobial and antifungal compounds and could protect feathers from microorganisms other than the B. licheniformis tested in this study. “Our findings really highlight the array of factors influencing feather coloration. It's not just about being bright," said Alexander Downie, postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology and former Princeton University graduate student. The authors say that whether bacteria actually pose a serious threat to feathers in the wild is an important area for future research. Ali ended, “This beauty and the bacteria story motivates us to ask new and exciting questions. Parrots have much to teach us about evolution and the natural world. I can’t wait to learn more from them.”
Journal of Experimental Biology
Experimental study
Animal tissue samples
Colourful psittacofulvin-containing parrot feathers do not resist bacterial degradation
12-Nov-2026