Study Sheds Light on Evolution of Plumage Coloration in African Starlings

African starlings, a diverse group of primarily brightly colored birds known for their metallic sheens, change color about ten times faster than their ancestors and even their modern relatives, according to a team of ornithologists writing in the Proceedings of the National Academy of Sciences.

A family of Superb starlings at Wilhelma Zoo in Stuttgart, Germany. The juvenile is in the middle of two adults (Dennis Irrgang / CC BY 2.0)

A family of Superb starlings at Wilhelma Zoo in Stuttgart, Germany. The juvenile is in the middle of two adults (Dennis Irrgang / CC BY 2.0)

“Many people enjoy bird watching because of all the different, beautiful colors of feathers, but this study gives us a closer look into the story of how these colors came to be, and how they changed over their millions of years of evolution,” said lead author Rafael Maia, a graduate student at the University of Akron.

The team used analysis of microscopic feather structures, spectral color analysis and evolutionary modeling to analyze patterns of evolution in African starlings.

The researchers focused mainly on the different forms of feather melanosomes, the parts of cells that carry the dark pigment melanin. Melanosomes are found in all vertebrates and most typically show up as black, brown or gray. But in birds, melanosomes are sometimes organized in a way that interacts especially with light to create colorful, metallic and often iridescent feathers. The melanosomes of the African starling groups evolved to become especially suited to react with light of various wavelengths, according to the researchers.

The ancestors of today’s starlings reached Africa about 17 million years ago. Back then, most likely had simple, rod-like melanosomes that are found in most bird species. As the ancient starlings of Africa started to split into new species, new melanosome types began to emerge. Some species retained the ancestral rods, while others shed them for one of three unusual forms of melanosomes: hollow rods, solid flattened rods and hollow flattened rods.

Some of the nearly 10,000 species of birds in the world also have some of these modified melanosomes, but – as a group – African starlings are the only birds to have all four types among them.

The Greater Blue-Eared starling (Paul Barnard)

The Greater Blue-Eared starling (Paul Barnard)

“With the appearance of these new shapes comes new ways of interacting with light, and a whole range of colors that these birds could never have made before can now be produced in their feathers,” Maia said.

These African starlings didn’t just leave their simple bland relatives behind slowly. When they took on the new melanosome types, they began to sport spiffier plumage more than ten times faster than their kin.

“Evolving these new melanosomes was like inventing the wheel for these birds – it allowed starlings to reach new colors at an incredibly fast rate,” explained senior author Dr Matt Shawkey, also from the University of Akron.

Similar key evolutionary patterns have been found in the jaws of African fishes and the beaks of finches that allow them to adapt rapidly to explore new food sources. But it’s not quite the same.

“Bright colors don’t help you get a meal – and they certainly don’t help hide you from a predator,” Dr Shawkey said.

“Feather coloration is very important in African starlings because it is used to signal quality and dominance when competing for mates,” explained co-author Dr Dustin Rubenstein of Columbia University in New York. “They help establish dominance and rank in females and males.”

“Like some other classic evolutionary innovations, these elaborate ornaments seem to have promoted diversification at a relatively fast rate,” Maia said. “However, they do so not by allowing you to eat something new, but just by changing your look.”

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Bibliographic information: Rafael Maia et al. Key ornamental innovations facilitate diversification in an avian radiation. PNAS, published online June 10, 2013; doi: 10.1073/pnas.1220784110