Gold is the new pink: iridescent hummingbirds flummox scientists with a colour change

When researchers found a hummingbird with shimmering gold feathers in the Peruvian Andes, they thought they’d found a new species.

But the truth turned out to be much weirder than that: a quirk of hybrid breeding, evolution and the physics of light.

The bird, as it turns out, is a hybrid of two different species: the Pink-throated Brilliant hummingbird, Heliodoxa gularis, and the Rufous-webbed Brilliant hummingbird, Heliodoxa branickii.

Both of these birds are native to western South America, and both have bright pink throats.

It’s very unusual for hummingbirds in the same species to have differently coloured throats, so when researchers stumbled across a specimen with a gold throat in the Cordillera Azul National Park, Peru, they assumed it belonged to neither group.

“I looked at the bird and said to myself, ‘This thing doesn’t look like anything else.’ My first thought was, it was a new species,” says Dr John Bates, a curator of birds at Chicago’s Field Museum, US, and senior author on a paper about the bird, published in Royal Society Open Science.

But when they checked the bird’s mitochondrial DNA, which comes just from the mother, they found it matched H. branickii. Then, looking at the rest of the bird’s DNA, they could see both H. branickii and H. gularis genes.

It wasn’t a 50/50 split between each species: it looks like one of the bird’s ancestors had an H. branickii and H. gularis parent, and then mated with more H. branickii hummingbirds down the line.

So how do two pink-throated hummingbirds produce one with a gold throat? It’s because of the way the feathers are coloured: rather than a simple pigment, they’re made by a thing called structural colour.

Structural colour is when a hue is created from the way light interacts with the micro- or nano-structures on a creature.

Hummingbird feathers specifically are iridescent: their colours come from light hitting the feather cells at different angles, being filtered and bent until it shines back out again at a different hue.

“There’s more than one way to make magenta with iridescence,” says lead author Dr Chad Eliason, a senior research scientist at the Field Museum.

“The parent species each have their own way of making magenta, which is, I think, why you can have this nonlinear or surprising outcome when you mix together those two recipes for producing a feather colour.”

The bird’s colour might help answer some questions about how hummingbirds evolve. We don’t know how common hummingbird hybrids are, but they might be a driver behind colour change.

“Based on the speed of colour evolution seen in hummingbirds, we calculated it would take 6-10 million years for this drastic pink-gold colour shift to evolve in a single species,” says Eliason.