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While birds often have vibrant plumage, the dazzling diversity in the colour of their eyes might be just as important for their survival and reproductive success, according to on the topic.

Many . The black-and-red-broadbill (Cymbirhynchus macrorhynchos), for instance, has sapphire blue or vivid emerald eyes. The Satin bowerbird (Ptilonorhynchus violaceus) has irises of periwinkle and purple. Some species of green pigeon (in the genus Treron) have magenta peepers and the bank cormorant (Phalacrocorax neglectus) sports a 鈥渟unset鈥� iris pattern, where the top half of its eye is orange and the bottom half is deep teal.

To try and understand why, at Louisiana State University and his colleagues analysed the results of more than 250 previously published bird studies.

One idea was that eye colour might aid bird vision and, as a result, boost their ability to find food. But the review suggests evidence for this hypothesis is lacking. 鈥淚t hasn鈥檛 been tested,鈥� says Corbett. 鈥淚t might [aid vision], but it also might not.鈥�

Instead, the studies generally point towards another important factor. 鈥淚 think the strongest driver of eye colour variation in birds is signalling,鈥� says Corbett. In other words, birds use eye colour to communicate with one another.

For instance, some of the studies Corbett鈥檚 team analysed demonstrated that male Queen Carola鈥檚 six-wired birds-of-paradise (Parotia carolae) . Other studies showed that some herons and pelicans . Meanwhile, one study in yellow-eyed penguins suggested that the .

Communication can be about intimidation as well as attraction. For instance, a of jackdaw behaviour revealed that the birds were more hesitant to approach a nest after researchers added an image of a jackdaw with bright eyes to it 鈥� but they weren鈥檛 deterred by the image of a jackdaw with darker eyes.

at the Spanish National Research Council, who wasn鈥檛 involved in the study, suspects there may be many factors behind the variation in bird eye colour. 鈥淭he rainbow of eye colours in birds must respond to different needs, and have different functions,鈥� he says. But the selective pressures influencing eye pigmentation remain to be studied in more than 98 per cent of bird species. 鈥淲e need to know as much as we can on eye structure, colour, and function to understand how these fascinating creatures confront the world they live in,鈥� says Negro.

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