Plenty of birds have iridescent feathers, but hummingbirds do it best.

From their ability to fly backwards to their impossibly diminutive size, we probably don't have to tell you that hummingbirds are special. Even within the bird world, these fluttering beauties stand out. Particularly when it comes to their coloration.

With more than 300 species, hummingbirds boast some of the most spectacular colors in the animal world. And, when the light hits them the right way, they shimmer like mother of pearl.

As Chad Eliason, an evolutionary biologist at the Field Museum of Natural History who studies hummingbirds, explained to Popular Science, plenty of birds have iridescent feathers, but hummingbirds do it best.  (OK, so maybe not this little guy.)

But how?

Eliason and his colleagues recently set out to discover the hummingbird's secret to first-rate iridescence by examining their melanosomes, microscopic packets of melanin pigment that create this iridescence effect in their feathers. The team reported their findings in the journal Evolution earlier this month.

In birds, melanosomes are found in little filaments called barbules, which branch off of the main barbs that line the shaft of a feather. Sort of like leaves on a tree branch. Humans also have melanin—it's what colors our hair, skin, and eyes—but our bodies use it in a much different way.

"We have kind of the same basic ingredients as birds do, but birds have just gone wild with layering and structuring those materials in different ways to play with light in unique ways," Eliason told Popular Science.

But when it comes to melanosomes, hummingbirds are the wildest of them all.

Examining barbules from 34 different hummingbird species under an electron microscope, Eliason and his team learned that the melanosomes in hummingbird barbules are flattened into a pancake shape. By contrast, the melanosomes in other animals are log shaped. What's more, hummingbird melanosomes are filled with tiny air bubbles, which create complex surfaces for light to bounce off of. And, as a result, iridescent brilliance.

The thickness of the melanosomes, the size of the air bubbles, and how many melanosomes were stacked on top of each other varied between species, which plays a role in the "explosive color diversification" seen in different hummingbird species.

"In some species you might have 15 layers of these air-filled pancakes," Eliason noted.

As for what that all means in terms of evolution and breeding, the scientists still have more work to do.

"There's this one case where you have two pink-throated species and when they breed the offspring is a bright yellow color, which is totally unexpected," Eliason told Popular Science. "We're studying now what different combinations [of color traits] are allowing hybrids to look so weird compared to their parental species."