On The Iridescent Feathers Of Birds

To address this question, Research Scientist Chad Eliason and a roster of international colleagues built a massive database of bird species to explore the spread of color across the globe. And as they now report in Nature Ecology & Evolution, they found that iridescent, colorful feathers originated 415 times across the bird tree of life, and in most cases, arose outside of the tropics–and that the ancestor of all modern birds likely had iridescent feathers, too. There are two main ways that color is produced in animals: pigments and structures. Cells produce pigments like melanin, which is responsible for black and brown coloration. Meanwhile, structural color comes from the way light bounces off different arrangements of cell structures. Iridescence, the rainbow shimmer that changes depending how light hits an object, is an example of structural color. Tropical birds get their colors from a combination of brilliant pigments and structural color. Much of Chad’s work has focused on structural color, so he wanted to explore that element of tropical bird coloration. He and his colleagues combed through photographs, videos, and even scientific illustrations of 9,409 species of birds—the vast majority of the 10,000-ish living bird species known to science—logging which species have iridescent feathers, and where those birds are found. They combined this data with a pre-existing family tree, based on DNA, showing how all the known bird species are related to each other, then fed the information to a modeling system to extrapolate the origins and spread of iridescence. Based on all this data, the modeling software determined the most likely explanation for the bird colors we see today: colorful birds from outside the tropics often came to the region millions of years ago, and then branched out into more and more different species. The model also suggests that the common ancestor of all modern birds, living 80 million years ago, had iridescent feathers. Chad was excited to learn that the ancestral state of all birds is iridescence. “We’ve found fossil evidence of iridescent birds and other feathered dinosaurs before, by examining fossil feathers and the preserved pigment-producing structures in those feathers,” he observed. “So,” he went on, “we know that iridescent feathers existed back in the Cretaceous. Those fossils support the idea from our model that the ancestor of all modern birds was iridescent too. We’re probably going to be finding a lot more iridescence in the fossil record now that we know to look.” But big questions remain, like why iridescence evolved in the first place. Notes Chad, “Iridescent feathers can be used by birds to attract mates, but iridescence is related to other aspects of birds’ lives too. For instance, tree swallows change color when the humidity changes, so iridescence could be related to the environment, or it might be related to another physical property of feathers, like water resistance. Knowing more about how there came to be so many iridescent birds in the tropics might help us understand why iridescence evolved.” For more, read Kate Golembiewski’s press release, which just broke this morning, and from which this blurb was condensed. The research has already been picked up by Popular Science and Scienmag, with more sure to come throughout the day. *  Co-authors include Michaël P.J. Nicolaï of Ghent University/Royal Belgian Institute of Natural Sciences, Cynthia Bom of Vrije Universiteit Amsterdam, Eline Blom of Naturalis Biodiversity Center, Liliana D’Alba of Ghent Univ./Naturalis Biodiversity Center, and Matthew D. Shawkey of Ghent Univ.
July 26. 2024