A study published today in the journal Evolution explains why hummingbird feathers are so iridescent; that is, why they shimmer in the light and shift as you look at the birds from different angles.
Other birds like ducks and grackles have iridescent feathers, of course, but hummingbirds take the trait to another level. Chad Eliason, a postdoctoral researcher at the Field Museum in Chicago, and an international team of colleagues conducted the largest-ever optical study of hummingbird feathers. They examined the feathers of 35 species with transmission electron microscopes and compared them with the feathers of other brightly colored birds, like green-headed Mallards, to look for differences in their make-up.
The key difference, the researchers say, are structures called melanosomes in hummingbird feathers. Ducks have log-shaped melanosomes without any air inside, but hummingbirds’ melanosomes are pancake-shaped and contain lots of tiny air bubbles. The flattened shape and air bubbles of hummingbird melanosomes create a more complex set of surfaces. When light glints off those surfaces, it bounces off in a way that produces iridescence.
The researchers also found that the different traits that make hummingbird feathers special — like melanosome shape and the thickness of the feather lining — are traits that evolved separately, allowing hummingbirds to mix and match a wider variety of traits. It’s kind of like how you can make more outfit combinations with three shirts and three pairs of pants than you can with three dresses. All in all, hummingbird feathers are super complex, and that’s what makes them so much more colorful than other birds.
“A good analogy would be like a soap bubble,” says co-author Matthew Shawkey of Belgium’s University of Ghent. “If you just look at a little bit of soap, it’s going to be colorless. But if you structure it the right way, if you spread it out really thin to form the shell of a bubble, you get those shimmering rainbow colors around the edges. It works the same way with melanosomes: with the right structure, you can turn something colorless into something really colorful.”
More wonderful questions to explore
And, the authors note, this project opens the door to a greater understanding of why hummingbirds develop the specific colors that they do. “Not all hummingbird colors are shiny and structural. Some species have drab plumage, and in many species, the females are less colorful than the males,” notes co-author Rafael Maia, a biologist and data scientist at Instacart.
“In this paper we describe a model of how all these variations can be achieved within feathers,” says co-author Juan Parra from Colombia’s Universidad de Antioquia. “Now other wonderful questions appear. For example, if it is possible to display a wide variety of colors, why are many hummingbirds green? Whether this reflects historical events, predation, or female variation in preferences are still open and challenging questions.”
“This study sets the stage for really understanding how color patterns are developed. Now that we have a better idea of how feather structure maps to color, we can really parse out which genes are underlying those really crazy colors in birds,” says Eliason.