By Jody Bourton
Earth News reporter
Looking good, a 'glossy' male great-tailed grackle
Male birds with glossy feathers are more attractive to the opposite sex.
Scientists found that male great-tailed grackles have glossier feathers than females and that glossier males attract more females.
The discovery comes from the first study to measure the glossiness of any animal's plumage or pelage.
Scientists report in the journal Behavioral Ecology and Sociobiology that it may help explain why some animals have evolved to be iridescent.
Researchers have long known that animals evolve traits that are sexually attractive to the opposite sex.
We see glossiness as a potential intermediate step towards the evolution of iridescence
Mr Matthew Toomey
Arizona state University, Tempe, US
For example, the healthiest male birds of paradise may be capable of growing the longest ornamental feathers, which then signal the male's health and vigour back to potential mates.
Many animals and birds are thought to be brightly coloured for the same reason.
Last year scientists also discovered that some large species of bird have silver wings, created by a previously unknown structure in their feathers, which they use to advertise their fitness.
But until now no-one has tried to measure whether animals also use glossiness, where their hair or feathers reflect back light like a mirror, to signal to the opposite sex.
To do so, PhD researcher Mr Matthew Toomey from Arizona State University, Tempe, US and colleagues examined wild great-tailed grackles (Quiscalus mexicanus), a type of blackbird.
The researchers captured the birds, photographed their plumage and measured the length of the birds' tails before releasing them.
Using a reflectance spectrophotometer and computer software, they then calculated the glossiness of the bird's feathers in each photograph.
"We found that male great-tailed grackles were significantly glossier than females," Mr Toomey says.
They also found that males with the glossiest feathers also had the longest tails.
That is important because tail length is known to be a sexually selected trait in these birds: males with longer tails reproduce more than males with shorter tails.
So glossier males likely mate with more females.
Further research on museum specimens of other North American bird species confirmed the pattern.
The researchers cannot be sure, but it could be that male birds use glossy feathers as well as long tails to signal their fitness to females.
"These results suggest to us that glossiness may play a role in visual signalling and glossier males may be more attractive and better competitors for mates," says Mr Toomey.
If true, that may also open up our understanding of the evolution of more complex optical phenomena, such as why some animals become so iridescent, a related property to glossiness where certain surfaces appear to change colour as the angle of view changes.
Birds and butterflies, for example, often have strikingly iridescent wings, and Mr Toomey's research suggests that sexual attraction may play some role in this.
"Glossiness depends upon material properties like the smoothness of a surface and its index of refraction," says Mr Toomey.
So in birds, "we can imagine a scenario where selection for glossiness can lead to a refinement of the microstructure of a feather," Mr Toomey explains.
"Then if you take this refinement one step further, adding levels of organisation, these same feather microstructures can produce iridescence."
"In this way, we see glossiness as a potential intermediate step towards the evolution of iridescence."