Parrots, toucans, and other brightly colored tropical birds are typically found exactly there–the tropics. Those that live further north and south tend to have more bland feathers. The origin of these differences in plumage has puzzled scientists for centuries: did colorful feathers evolve in the tropics or do tropical birds have colorful ancestors that flocked to the region from somewhere else?
A new dive into the bird family tree found that iridescent colorful feathers originated 415 times. In most cases, they arose outside of the tropics. It’s also likely that the ancestor of all modern birds had iridescent feathers. The findings are detailed in a study published July 26 in the journal Nature Ecology & Evolution.
Color and iridescence in the animal kingdom
One way animals produce color is by pigments produced by the cells. In birds, the pigment melanin generates black, gray, brown, and orange colors. Carotenoids are used by specialized feather structures to create those brighter hues.
The other process is through structural color. This comes from the way that light bounces off different arrangements of cell structures. Iridescence–that rainbow shimmer coming off of a soap bubble–is an example of structural color.
[Related: Why blue animals are so rare in nature.]
“Iridescence works by filtering out some wavelengths of light but reflecting others back to your eye,” Chad Eliason, a study co-author and research scientist at the Field Museum in Chicago, tells Popular Science. “Because the color you see depends on the distance traveled by the light through the iridescent object, when you change your viewing angle you see different colors. That’s the phenomenon of iridescence.”
Tropical birds get their bright colors from a mix of brilliant pigments and structural color like iridescence.
‘We did a lot of math’
In the new study, the team built a database of 9,409 species of birds to look at how color spread around the world. The team combed through videos, photographs, and scientific illustrations of most of the living bird species that have been scientifically documented. Along the way, they kept track of which species have iridescent feathers, and where those birds are located.
They then combined the data on bird coloration and a pre-existing family tree that is based on DNA. This helped them determine how all of the species are related to each other. According to Eliason, “we did a lot of math,” and fed the information into a modeling system to pinpoint the origins and spread of iridescence.
The model indicated that colorful birds from outside the tropics came to the region millions of years ago. They then branched out into numerous different species over time. A surprise was also in all of that math.
A colorful surprise
Living birds are technically a specialized group of dinosaurs, and the model indicated that the common ancestor of all the birds alive today had the iridescent feathers that still glitter across the bird family tree.
The earliest known bird–Archaeopteryx–lived 140 million years ago. A sub-group of birds called Neornithes evolved about 80 million years ago and became the only birds to survive the mass extinction 66 million years ago that wiped out most other dinosaurs. Modern birds are members of Neornithes and the model indicates that iridescent feathers go back this far.
[Related: Leggy dinosaur species could be the latest feathery clue to bird evolution.]
Scientists search for evidence of iridescence by looking for the structures that cause iridescence in modern birds.
“In this case, the structures are layers of melanin pigment in feathers,” says Eliason. “If we see similar layers or structures in the fossils, we can assume those extinct animals could make iridescence as well.”
This finding could have important implications in paleontology, but there is still a lot more work to do to learn more.
“Next steps are to study where on the body iridescence was found in ancestral birds, why some birds use iridescence structural color and others non-iridescent structural color for signaling, and also to study the biological function of iridescence to understand why it is associated with dispersal/expanding ranges across birds as we found,” says Eliason.
