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Eyes may explain why bird plumage ‘pops’

MONASH U. (AUS) — Varying ability to see UV light may account for birds’ wild diversity of color, such as the brilliant blue plumage that male fairy-wrens use to stand out from their surroundings.

Birds have more sophisticated visual systems than humans, and they can see certain wavelengths of light—the ultraviolet (UV)—that are invisible to us.

Such a developed visual system is often used as an explanation for the large diversity of bird colors, as color signals of animals are often finely tuned to their visual abilities.

But there is also variation among birds in how they perceive colors and this may affect which colors evolve.

Color vision in most birds tends to fall into two categories, either violet-sensitive (V-type) or ultraviolet-sensitive (U-type). Both types are sensitive to the UV, but U-type eyes have higher sensitivity to UV and blue light.

Accordingly, many species with U-type vision often show UV/blue plumage, presumably due to better perception of such colors by U-type vision. However, the idea that U-type birds do actually perceive UV/blue colors better has never been fully explored.

Now in a study published in the Proceedings of the Royal Society B, researchers have provided evidence of this widespread idea using fairy-wrens to explain how the vibrant blue plumage found on the breeding male of several species could be linked to higher visual sensitivity to ultraviolet (UV).

Visual sensitivities among different species of fairy-wren can be either V- or U-type, and previous work shows that the latter is only found in fairy-wren species with UV/blue plumage coloration.

Monash University ornithologist Kaspar Delhey, a research fellow at the School of Biological Sciences, and his colleagues believe a correlation between plumage coloration and U-type eyes in fairy-wrens is explained by the greater ability of this visual type to detect contrast between UV/blue plumage coloration and their natural environment.

Delhey says that preference by potential mating partners for plumage that contrasted greatly to the natural environment may have driven this correlation.

“The reproductive success of males can depend a great deal on plumage coloration and just how conspicuous or ‘good-looking’ the male doing the courting is, something that might be preferred by females,” Delhey says.

“We found that higher UV sensitivity increased the contrast of the blue plumage against the natural environment, possibly making these colors more attractive.

“In comparing the performance of both types of visual systems in different fairy-wren species, we were able to demonstrate that U-type generally outperform V-type visual sensitivities at detecting plumage colors against natural backgrounds and that this difference is particularly large for UV/blue colors.”

This may explain why all fairy-wrens with U-type eyes show UV/blue plumage colors when in breeding plumage.

Delhey says the study provided scope for further research to understand the role of differences in color vision in shaping the diversity of animal colors.

Delhey is also associated with the Max Planck Institute for Ornithology in Germany and worked with colleagues from the University of East-Anglia and Melbourne University on the study.

Source: Monash University

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