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.
Vibrant parrot feathers are surprisingly susceptible to bacterial attack, according to a study published on Thursday in the Journal of Experimental Biology.
While melanin-pigmented black feathers resisted degradation from keratin-eating bacteria, feathers containing psittacofulvins broke down the most in new laboratory tests. Researchers at Princeton University and other institutions revisited a 2010 hypothesis suggesting that these unique pigments protected parrots from microbial damage.
Most birds use carotenoid pigments to produce bright yellows, oranges, reds, and pinks. Parrots instead evolved psittacofulvins—a group of pigments found in no other animals on the planet—to achieve the same hues.
The 2010 study tested a small sample of parrot feathers using the bacteria Bacillus licheniformis, which is ubiquitous in wild birds. That earlier work indicated red feathers with high concentrations of psittacofulvins were resistant to bacterial attack, much like black feathers.
For the new study, Ali et al. (2026) sampled thousands of parrot feathers from closely related species to control for natural variations in feather properties. The team placed the feathers in a liquid solution inoculated with B. licheniformis, then quantified the small proteins released as the bacteria fed on the feather keratin.
The least degraded feathers were black ones containing high amounts of melanin, which is known to reduce damage from keratin-eating bacteria and make feathers physically stronger. By contrast, red, orange, yellow, and green feathers—all containing psittacofulvins—suffered the most breakdown.
Feathers lacking psittacofulvins, such as blue, white, black, and grey varieties, fared substantially better in the tests. The results contradicted the research team’s initial expectations.
“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”.
Jarome R. Ali, lead author and former Princeton graduate student
Ali, who is now a postdoctoral researcher at New York University, said that red feathers should have stayed intact based on previous assumptions. Instead, he explained, “they had totally fallen apart, while other colors, like blue, seemed mostly unbothered.”
The researchers propose that incorporating psittacofulvins into feather keratin might slightly destabilize its structure. This alteration could make the structure more vulnerable to the enzymes that bacteria use to break it down.
“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.
Mary Caswell Stoddard, professor of ecology and evolutionary biology at Princeton
Stoddard summarized the evolutionary trade-off as beauty over bacteria, noting that parrots retain red, orange, yellow, and green hues despite their susceptibility to microbial attack. Alternatively, researchers suggest psittacofulvins might serve undiscovered roles, noting that their chemical structure resembles known antimicrobial and antifungal compounds that could defend against other microorganisms.
“Our findings really highlight the array of factors influencing feather coloration. It’s not just about being bright,” said Alexander Downie, a postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology and former Princeton graduate student.
The study authors emphasize that whether bacteria pose a serious threat to feathers in the wild remains an important area for future investigation.
