Researchers have previously observed that iridescence - a phenomenon in which the colour of a material changes as the angle of light changes - is found across the dinosaur family tree, as well as in modern birds. However, it was unclear how iridescence evolved, and how it is linked to the evolution of feathers.
The new study, published in Nature Communications, finds that the microstructure of the tiny, barb-like branches that feathers are made of plays a crucial role.
Lead author Dr Mostafa Kasim from Bristol's School of Earth Sciences said: "We discovered that barb shape is a key factor in determining whether or not a feather is iridescent. Iridescence is caused by the way barb ridges scatter and interfere with light, generating structural colours that change as the angle of the feather is altered."
To understand the evolutionary link between barbs and iridescence, the researchers took advantage of the fact that different regions within a single feather can have distinct barb shapes.
They analysed feathers from 45 modern and extinct bird species, including chickens, peacocks, ducks and hummingbirds, and used a technique called Fourier transform light microscopy to measure barb shapes.
Their results show that there are two key groups of iridescent barb shapes. One group, found only in modern birds, produces short-wavelength, blue to violet iridescence. The other group, found in modern birds as well as Archaeopteryx, is responsible for longer wavelength, green to red iridescence.
Dr Kasim said: "Our study highlights a connection between the evolution of iridescent barb shapes and the evolution of birds themselves, particularly within the 'neoaves' group, which includes approximately 95 percent of modern bird species. This group saw the diversification and radiation of colourful plumages that led to the dazzling array of iridescent colours seen in many bird species today."
Dr Kasim also notes that the same physical mechanism that generates iridescent colours in feathers can be found in a variety of other animal structures, such as the scales of tropical butterflies, the carapaces of beetles, the skin of frogs, and the shells of some molluscs.
"Therefore," he said, "our study not only advances our understanding of the evolution of iridescent plumage in birds and their dinosaur ancestors, but also provides insights into the evolution of structural colouration more broadly in the animal kingdom."
