'˜Trick of the light' could explain colourful bird plumage

A trick of the light could explain how birds evolved their colourful feathers, scientists say.

A Masked Trogon in Ecuador.

The phenomenon of iridescence, in which luminous colours appear to change when seen from different angles, is responsible for some of the most striking visual displays in the animal kingdom, according to a study of feathers from almost 100 modern bird species.

A team of researchers from Bristol University used electron microscopy scanning to examine the feathers of birds with iridescent plumage.

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The samples, from the collections of the Zoological Museum of Copenhagen, contain melanosomes – animal cell structures responsible for trapping, synthesising, storing and moving the light-absorbing pigment melanin, and for colour and protection from sunlight.

The study showed that iridescent feathers contain the most varied melanosome of all types of bird coloration sampled to date.

Unlike black, grey and brown feathers that always contain solid melanosomes, iridescent feathers can contain melanosomes that are hollow or flattened.

Klara Norden, the study’s lead author, who conducted it during her undergraduate years, said: “It is already known that structural coloration is responsible for 70 per cent of the colour variability in birds.

“These two facts might be coupled – birds evolved varied forms of melanosomes to achieve ever greater diversity in colour.

“I wanted to find out if we could improve current predictive models for fossil colour based on melanosome morphology by including all types of melanosomes found in iridescent feathers.”

Dr Jakob Vinther, co-author of the study and a leading researcher in the field of paleocolour at Bristol, had already collected the perfect fossil samples to test the new model on.

He said: “We had sampled Scania Cypselus, related to modern tree swifts, and Primotrogon, ancestor to modern trogons.

“These groups are iridescent today. Did their 48-million-year-old ancestors from Germany also have iridescent plumage?”

The model predicted that Primotrogon was probably iridescent, but it used solid rather than hollow melanosomes, unlike its modern descendants.

“This demonstrates how we now have the tools to map out the evolution of iridescence in fossil lineages,” said Ms Norden.

“It opens the door to many new discoveries of dazzling displays in fossil birds and other dinosaurs.”