Pink + pink = gold? The feathers of a hybrid hip differ from those of its parents

Not surprisingly, the brilliant pink-throated humpback, Heliodoxa gularis, has a stunning pink throat. So does its cousin, the brilliant hairy humpback, Heliodoxa branickii. When scientists discovered a Heliodoxa humpback with a glittering gold throat, they thought they might have discovered a new species. However, DNA revealed another story: The golden-throated bird was a hybrid of the two pink-throated species.

John Bates, senior author of a new study in the journal Royal Society Open Science Reporting on the hybrid, the unusual bird first appeared while doing fieldwork in Peru’s Cordillera Azul National Park, which protects the outer ridge on the eastern slopes of the Andes mountains. Since the area is isolated, it would make sense to have a distinct population. “I looked at the bird and said to myself, ‘This thing doesn’t look like anything else.’ My first thought was that it was a new species,” says Bates, curator of birds at Chicago’s Field Museum.

When Bates and his colleagues collected more data about the specimen in the Field Museum’s Pritzker DNA Lab, however, the results surprised everyone. “We thought it would be genetically distinct, but it matched Heliodoxa branickii in some markers, one of the pink-throated hummingbirds from that general area of ​​Peru,” says Bates. If it was H. branickii, the bird would not have had golden throat feathers; in the hummingbird family, members of the same species rarely have different throat colors.

The first run of DNA sequencing looked at mitochondrial DNA, a type of genetic material that is only passed down through the mother. That mitochondrial DNA yielded a clear match to H. branickii; the researchers then analyzed the bird’s nuclear DNA, including contributions from both parents. This time, the DNA showed similarities to H. branickii and its cousin, H. gularis. He was not half branickii and half gularis, however – one of his ancestors must have been half and half, and then later generations mated with more branickii birds.

The question remained how two species of pink-throated bird could produce a non-pink hybrid. The study’s first author, Field Museum senior research scientist Chad Eliason, says the answer lies in the complex ways in which redescent feather colors are determined.

“It’s a bit like cooking: If you mix salt and water, you know what you’re going to get, but mixing two complex recipes together can have more unpredictable results,” says Eliason. “This hybrid is a combination of two complex recipes for a feather from its two parent species.”

Feathers get their basic color from pigments, such as melanin (black) and carotenoids (red and yellow). But the structure of feather cells and the way light bounces off them can also produce what’s called structural color. Color-shifting iridescence is the result of structural color.

The researchers used an electron microscope to examine throat feather structure at a subcellular level, and an analytical technique called spectroscopy to measure how light bounces off the feathers to produce different colors. They found subtle differences in the color origins of the parents, which explains why their hybrid offspring produced a completely different color.

“There’s more than one way to do magenta with blur,” says Eliason. “Each parent species has its own way of making magenta, which I think is why you can have this non-linear or surprising result when you mix those two recipes together to produce a feather color .”

While this study helps explain the strange coloration of one unusual bird, the researchers say it opens the door to more questions about hybridization.

Distinct species are generally defined as lines that are genetically distinct and do not interbreed; hybrids break that rule. Sometimes unusual hybrids are one-off or sterile, like mules; in other cases, hybrids can form new species. It is not clear how common hip hybrids like the one in this study are, but the researchers speculate that hybrids like this may contribute to the diversity of structural colors found throughout the hip tree.

“Based on the speed of color evolution seen in hummingbirds, we calculated that it would take 6-10 million years for this rose gold color change to develop in a single species,” says Eliason.

Co-author Mark Hauber at the University of Illinois Urbana-Champaign says that “This study gives us clues about the nanostructural basis of evolutionary changes in color.”