Ultraviolet Light Reveals Fluorescent Pigments in Long-Eared Owl Feathers

While the visible diversity in wildlife traits is easy for humans to observe, new research from Drexel University’s College of Arts and Sciences suggests there may be even more variation that we simply can’t see.

A study published in The Wilson Journal of Ornithology has identified fluorescent pigments in the feathers of Long-eared Owls—visible to humans only under ultraviolet (UV) light.

Led by Emily Griffith, a PhD candidate in Drexel’s Biodiversity, Earth & Environmental Science department, the research reveals that these pigments can vary within a single population of owls. This variation may hold clues to the purpose and function of these unique feather traits.

To explore this, the team used a fluorometer—a tool that measures light emitted by a substance after it absorbs radiation like UV light—to analyze feather samples from Long-eared Owls migrating through Michigan’s Upper Peninsula in spring 2020.

“We are only beginning to describe fluorescent pigments in birds and other vertebrates,” Griffith explained. “While it’s important to identify which species have them, understanding their function means also looking at how they vary within a species—like the Long-eared Owl.”

Griffith noted that in many bird species, males use pigments to attract mates, which feeds the common belief that males are more colorful than females. But the findings from this study challenge that assumption.

“Our study shows female Long-eared Owls actually have much higher concentrations of fluorescent pigments in their feathers,” she said. “This questions the idea that colorful plumage is primarily a male trait. What’s more, this pigment doesn’t follow a simple male/female divide—it exists on a spectrum, influenced by a combination of age, sex, and body size.”

The research team believes that these pigments have likely been present in animals for a long time, but the tools to detect and study them have only recently become available.

Their interest in fluorescent plumage was partly sparked by how owl researchers use these glowing feathers to determine a bird’s age—the intensity of fluorescence fades over time.

Griffith emphasized that scientists are just beginning to uncover these hidden traits in owls and other birds, asking questions about what the fluorescence means, where it occurs, how it evolved, and why it’s there.

“We know so little about fluorescent pigments in bird feathers—and it’s not just owls that have them. It’s a really exciting time to study bird plumage.”