If you talk about a starling, most people in Europe and North America will picture a small bird with glossy black plumage. But that’s the common starling. It’s just one of 113 starling species, many of which have far more spectacular feathers. Just take a look at the selection above.
These resplendent plumes don’t just catch the eye. They may also explain why these birds are so diverse. According to a new study from Rafael Maia at the University of Akron, the starlings’ colours have made them more evolvable, accelerating their split into more and more species.
Many birds produce beautiful feathers using pigments that selectively absorb and reflect different colours of light. But starlings owe their most stunning colours to the structures of the feathers themselves.
As light hits the feathers, it encounters several layers. At each one, some light gets reflected and the rest passes through. If the layers are evenly spaced, the reflected beams amplify each other to produce exceptionally strong colours, which can easily change depending on the distance between the layers or the angle they’re viewed from. This effect is called iridescence. You can see it on the vivid throats of hummingbirds, the tail feathers of peacocks and the plumage of many starlings.
The layers mostly consist of small pigmented structures called melanosomes, which are found in all bird feathers. In their simplest form, they’re shaped like solid rods. But the starlings have added three types of deluxe features on top of this basic model. Some have evolved flatter melanosomes, which lets them pack more layers into the same space. Others have hollow melanosomes, which provide even more layers as light passes through solid walls and empty interiors. Yet others have melanosomes that are hollow and flattened.
When Maia thought about these structural colours, he was struck by how easy it would be for a starling to evolve a completely different palette. “It just needs to change how thick the layers are, or how spaced apart they are,” he says. By contrast, it would be very difficult to start making new pigments. You’d need ways of ushering the right starting ingredients through a new set of chemical reactions—the equivalent of building an entirely new factory just to make a car of a slightly different model. To evolve a new structural colour, you just need to rearrange the parts a little.
By looking at the starling family tree, Maia found that the basic melanosomes have evolved into the three complex types many times over. And although the complex types can change between themselves, they never revert back into the original solid rods. “I thought that maybe you’d have a lot of changing back-and-forth,” he says, “but actually, once these complex structures evolve, they stick.”
As the melanosomes moved from simple to complex shapes, the starlings’ colours became around 80 percent brighter. Their palette also expanded, which you can see in the image below. Each dot represents the colour of a starling feather, and species with complex melanosomes (red, blue and green dots) carry a broader range of colours than those with solid ones (purple dots). “Once you have a certain evolutionary step, you open up the range of colours you can produce,” says Maia.
But the starlings with hollow or flattened melanosomes don’t just have a more diverse palette—they also have a palette that diversifies faster. Maia found that their colours change at 10 to 40 times the rate of their cousins with simple melanosomes. They also produced new species at the fastest rates.
This all makes sense. Once they evolved the more complex melanosomes, the starlings could produce dramatic differences in colour by tweaking tiny details, like the thickness of the walls or the density of the layers. Natural selection suddenly had more variation to tinker with. And since starlings rely on colours to recognise each other and choose their mates, their fast-changing palettes would more quickly accentuate the differences between separate populations accelerating their split into new species.
This probably wouldn’t have happened if the starlings’ colours were produced by pigments. Consider a canary. It gets its yellow colour from pigments called carotenoids, which play important roles in their immune systems. If an individual can cope with shunting these substances into its feathers, it must be in good health, which makes the colour an honest sign of quality. A sickly canary simply cannot afford to be as yellow as a healthy one. But once the bird uses this honest signal, it’s hard to evolve a new one that is equally truthful. It would need to produce an entirely new set of chemicals that are both visible to an onlooker and tied to the bird’s health.
Starlings also use their structural colours as honest signals. Healthy, well-fed birds produce stronger iridescent colours than sickly, starving ones. But these birds prove their quality by making an even template. It doesn’t matter if their melanosomes give off a blue or purple or green iridescence, as long as they built to the same specifications and arranged evenly. The structure matters. The strength of the iridescence matter. The colour does not. This means that starlings can make an entire spectrum of colours that are all equally honest. They were free to diversity into ever more resplendent forms without sacrificing the reliability of their messages within their plumage.
Reference: Maia, Rubenstein & Shawkey. 2013. Key ornamental innovations facilitate diversification in an avian radiation. PNAS http://dx.doi.org/doi/10.1073/pnas.1220784110