In a Princeton laboratory, the old idea that the brilliant reds and yellows of parrots were armor against feather-eating microbes has begun to crack. Researchers watching a common keratin-digesting bacterium work through thousands of feathers found that the very pigments parrots evolved to paint themselves with, unlike anything else in the animal world, made feathers more easily undone. Black feathers, shielded by melanin, held firm, while the bright colors proved to be a quiet liability. It is a reminder that in nature a beautiful surface is often a negotiated compromise, where the pull towar
Parrots' Vibrant Feathers Prove Surprisingly Vulnerable to Bacterial Attack
Beauty over bacteria
Mark
So the headline is that the colorful feathers are the ones bacteria wreck?
Mimi
Yes. Red, orange, yellow and green feathers, all colored by psittacofulvins, broke down the most. Blue, white, gray and black did much better, and black was the best.
Mark
Why did anyone expect the opposite?
Mimi
A 2010 study found red feathers resisted bacteria about like black ones. Melanin was already known to help, so protection from psittacofulvins seemed plausible.
Luke
What changed between the two studies, though? Is the earlier one simply wrong?
Mimi
The authors say the difference likely comes from sampling. The earlier work used a relatively small set of feathers; this one used thousands and more species. That's their explanation, not something they've proven.
Luke
And the destabilized-keratin idea? Is that demonstrated?
Mimi
No, it's a suggestion. The source says the researchers propose it as a possible reason.
Mark
So what does it say about evolution?
Mimi
Stoddard's reading is that selection for vivid color outweighs the bacterial cost. Beauty over bacteria.
Luke
But that rests on bacteria being a real threat to wild birds, and the authors say that's unknown. They also tested only one bacterium, so other microbes could behave differently.
Mimi
Right. Psittacofulvins resemble some antimicrobial compounds, so the story may not be finished.
The Pulse
- A 2010 hypothesis that parrot pigments shielded feathers from bacteria is now contradicted by a far broader sampling of species.
- Red, orange, yellow and green feathers fell apart fastest in the bacterial bath, while black feathers resisted most strongly.
- Researchers suspect psittacofulvins weaken feather keratin's structure, leaving it open to the enzymes bacteria use to digest it.
- Scientists are weighing whether vivid color's bacterial cost is a trade-off parrots accept, or whether these pigments guard against other microbes still untested.
- The real-world danger to wild parrots remains unmeasured, and the question is now the next thing the field must answer.
In a Princeton laboratory, the old idea that the brilliant reds and yellows of parrots were armor against feather-eating microbes has begun to crack. Researchers watching a common keratin-digesting bacterium work through thousands of feathers found that the very pigments parrots evolved to paint themselves with, unlike anything else in the animal world, made feathers more easily undone. Black feathers, shielded by melanin, held firm, while the bright colors proved to be a quiet liability. It is a reminder that in nature a beautiful surface is often a negotiated compromise, where the pull toward display can outweigh the cost of decay, and whether that cost ever threatens a wild bird remains a question still waiting in the forest.
At Princeton, researchers filled a solution with Bacillus licheniformis, a bacterium common in wild birds that feeds on keratin, the protein feathers are made of. As the microbes ate, they released small proteins into the liquid, giving the team a precise measure of how much each feather had been destroyed. The results turned a long-held assumption on its head.
Parrots are unusual among birds. Where most species get their reds and oranges from carotenoids, parrots rely on psittacofulvins, pigments found in no other animal. A 2010 study had proposed that these pigments might protect feathers from keratin-eating bacteria, a meaningful idea, since feathers serve flight, temperature control, signaling and camouflage. That earlier work relied on a small set of feathers and suggested red ones held up about as well as black.
The new study, led by Princeton biologists and published in the Journal of Experimental Biology, drew on a much wider range of feathers from closely related species, which helped control for other differences. Black feathers, rich in melanin, degraded least. But red, orange, yellow and green feathers, all containing psittacofulvins, broke down the most, while blue, white, black and gray feathers fared better. Jarome R. Ali, the lead author, said the red feathers had "totally fallen apart" when he expected them to be intact.
The team suspects that building these pigments into keratin subtly destabilizes the structure. Senior author Mary Caswell Stoddard put it bluntly: "Beauty over bacteria." Yet the authors also note that psittacofulvins resemble known antimicrobial compounds, so they may guard against other microbes or serve purposes not yet understood. Alexander Downie added that feather color is shaped by many forces, not brightness alone.
What remains unknown is how much this matters beyond the lab. Whether bacteria seriously threaten feathers in wild parrots is an open question the researchers hope to answer next.
In a lab at Princeton, researchers placed thousands of parrot feathers into a solution seeded with Bacillus licheniformis, a bacterium common in wild birds that feeds on keratin, the protein feathers are made of. As the bacteria ate, small proteins leaked into the surrounding liquid, giving the team a way to measure how much of each feather had been destroyed. The result ran against what the field had come to expect.
Parrots are a puzzle in bird coloration. Most birds get their reds, yellows, oranges and pinks from carotenoid pigments. Parrots instead evolved psittacofulvins, a group of pigments found in no other animal, to produce those same hues. A pioneering 2010 study proposed a reason: psittacofulvins might shield feathers from keratin-eating bacteria. That mattered because feathers do a great deal, from flight and temperature control to signaling and camouflage, so damage from bacteria could carry real costs for a bird's fitness.
The idea had a logical neighbor. Melanin, the pigment behind dark colors in many animals, is already known to make feathers physically stronger and to reduce damage from keratin-eating bacteria. The 2010 work used a relatively small set of parrot feathers and the same B. licheniformis, and found that red feathers, rich in psittacofulvins, resisted attack about as well as black ones.
The new study, published in the Journal of Experimental Biology and led by Princeton biologists, used a much wider range of feathers from more species. By drawing on closely related parrot species, the team could control for differences in feather properties that might otherwise muddy the comparison.
Black feathers were the least degraded, which fit with their high melanin content. But the red, orange, yellow and green feathers, all of which contain psittacofulvins, broke down the most. Feathers without those pigments, meaning blue, white, black and gray, did substantially better.
"We thought psittacofulvins would combine with melanin to protect feathers, so our findings were entirely unanticipated," said lead author Jarome R. Ali, a former Princeton graduate student now a postdoctoral researcher at New York University. He said the red feathers "should have been intact, but they had totally fallen apart," while blue ones seemed mostly unbothered. He attributes the difference from the earlier study to the broader and more extensive sampling, and added that parrots often seem to break the rules.
The team offers a possible explanation for the weak protection. Building psittacofulvins into feather keratin could slightly destabilize its structure, leaving it more exposed to the enzymes bacteria use to break it down. Senior author Mary Caswell Stoddard, a Princeton professor of ecology and evolutionary biology, framed the trade-off this way: the pressures favoring vibrant color in parrots appear to outweigh the cost of bacterial degradation. "Beauty over bacteria," she said.
The authors also leave room for another possibility. Psittacofulvins have a chemical structure similar to some known antimicrobial and antifungal compounds, so they might protect feathers against microorganisms other than the one tested here, or serve some function not yet discovered. Alexander Downie, a postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology and a former Princeton graduate student, said the findings show that feather color is shaped by many factors: it is not just about being bright.
What remains open is how much any of this matters outside the lab. The authors say that whether bacteria pose a serious threat to feathers in wild parrots is an important question for future research.
Notable Quotes
We thought psittacofulvins would combine with melanin to protect feathers, so our findings were entirely unanticipated.— Jarome R. Ali, lead author
Beauty over bacteria: parrots are often red, orange, yellow and green even though these colors are quite susceptible to bacterial attack.— Mary Caswell Stoddard, senior author, Princeton
Want the full story?
Read the original at Phys.org ↗