Hidden glow in owl feathers reveals biological differences

Birdwatchers and scientists alike have long admired the stunning diversity of bird feathers. Their hues and patterns offer more than just visual delight – they tell stories of evolution, survival, and social cues. Yet beyond what we can see lies a hidden world of light.

A recent study from Drexel University and Northern Michigan University reveals that some birds like owls hold secrets invisible to human eyes.

The researchers have uncovered fluorescent pigments in the feathers of long-eared owls. These pigments glow only under ultraviolet (UV) light, revealing a new layer of complexity in avian plumage. While earlier studies focused on visible coloration, this study dives into traits concealed in darkness.

The discovery doesn’t just show that owls fluoresce – it demonstrates that this trait varies with age, size, and sex. These findings challenge assumptions about bird biology and open up questions about what fluorescence might truly mean.

Glowing pigments in owl feathers

The team, led by PhD candidate Emily Griffith, studied 99 long-eared owls captured at the Whitefish Point Bird Observatory in Michigan during the spring of 2020. They collected feathers from each bird and used a fluorometer to measure pigment levels.

The pigments in question – mainly coproporphyrin III and protoporphyrin IX – respond to UV light by emitting a visible glow. Normally hidden to the naked eye, this glow can appear pinkish under blacklight. These pigments degrade with time and sun exposure, making them useful for determining a bird’s age.

Griffith and her colleagues extracted the pigments using a specialized chemical process, then measured their fluorescence. They also recorded data on each owl’s weight, wing length, tail length, and visible plumage color.

Some owl feathers glow more

The results showed that age and sex were the strongest predictors of pigment concentration. Older birds had significantly more pigment than younger ones. Likewise, females had more pigment than males.

“We are only beginning to describe fluorescent pigments in birds and other vertebrates,” said Griffith. “Although describing what species they are present in is important, in order to understand what their function is we need to also describe how they vary within a species like the Long-eared Owl.”

Plumage color, which helps determine an owl’s sex, also matched pigment levels. Darker birds (typically female) showed more fluorescence than lighter ones (typically male). Owls with intermediate coloration fell in between, suggesting a spectrum rather than a binary trait.

Statistical tests confirmed that adult birds consistently had more pigment than juveniles. This trend held regardless of feather type or collection date, reinforcing the idea that pigment concentration reflects biological age and sex.

Body size affects pigment levels

The researchers also explored the relationship between body size and pigment levels. They found that weight positively correlated with pigment in males and small females. However, in larger females, this trend disappeared.

This suggests that there may be a saturation point – where adding more size doesn’t result in higher pigment levels. In other words, larger birds may already have as much pigment as their feathers can hold.

These findings highlight the complexity of traits that appear simple at first glance. Size matters, but only under specific biological conditions.

Rethinking the purpose of pigment

It’s long been thought that bright plumage helps males attract mates. Griffith challenges that narrative.

“Our study shows that female Long-eared Owls have a much higher concentration of these pigments in their feathers, challenging a common misconception that colorful plumage is a ‘male’ trait,” said Griffith.

“Moreover, this trait doesn’t follow a strict binary – the amount of fluorescent pigments in these owls exists on a spectrum where the amount of pigment is related to size, age and sex all together.”

Because males perform courtship flights, one might expect them to have more pigment. Yet the data shows the opposite. That raises questions about whether fluorescence in these owls even serves a visual signaling purpose.

Pigment and heat retention

Rather than attraction, the team suggests a different function for owl feathers: thermal regulation.

Porphyrins, the class of pigments studied here, are known to absorb and reflect radiation. In bird eggs, protoporphyrin IX helps retain heat.

The researchers believe a similar mechanism might apply to female owls, who spend more time incubating eggs. More pigment could mean better heat retention during cold spring nights.

This theory would explain why females have more pigment. Males, who hunt and move often, wouldn’t benefit as much from this trait.

Understanding why owl feathers glow

“So little is known about fluorescent pigments in bird feathers and owls aren’t the only ones with fluorescent pigments,” said Griffith. “So, it’s a really exciting time to be interested in studying bird plumage.”

Their study urges scientists to reconsider how feather traits evolve. Future work could explore how pigment production interacts with other compounds like melanin. It could also look at how environmental factors shape feather chemistry.

If these hidden pigments carry biological meaning, we need better tools to understand them. Questions remain about how quickly they fade, how owls perceive them, and whether they play roles in communication, survival, or both.

The glowing feathers of long-eared owls are not just a scientific curiosity. They are a reminder that much of nature’s beauty lies beyond the spectrum of human vision.

The study is published in the journal The Wilson Journal of Ornithology.

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