Some birds really go to town on their appearance. From elaborate trains to tufty crowns, a selection of our feathered friends really turn up the contrast. Although common mynas (Acridotheres tristis) are not members of the most vivacious end of the spectrum, they do splash out on a vivid pair of yellow spectacles. ‘The patch of yellow skin around the eyes is coloured by carotenoids, pigments that create yellow to red coloration in animals’, says Chloe Peneaux from the University of Newcastle, Australia, explaining that the birds obtain carotenoids from their diet. But it can take many other factors, in addition to a gaudy tint, to brighten a bird's appearance. ‘Fats, carbohydrates and proteins in foods are essential to help with important processes like growth and reproduction’, she says. Wondering how necessary these elements are for the bird's spectacular patches, Peneaux, Andrea Griffin and their colleagues fed local myna birds on a range of diets, including an option where they could select how much protein, carbohydrate and fat they consumed, to find out how important other nutrients are for the colour of their vivid patch of skin.
First, the team had to reset the birds’ eye patches to washed-out white by simply feeding them dog pellets, ‘a diet which apparently does not contain dietary carotenoids’, Peneaux explains. As soon as the mynas’ eyepatches were sufficiently faded, Peneaux and colleagues offered some birds a selection of three types of pellet, enriched with either protein, fat or carbohydrate, so that the birds could select how much of each nutrient they consumed. Others received the pellet selection and carotenoid-spiked water, while a third group was provided with carotenoid-supplemented water in addition to a dog food diet. Then, Peneaux monitored the colour of the birds’ eye patches, in addition to keeping track of the amount of protein, fat and carbohydrate that they consumed and the amount of carotenoid in their blood.
Over the course of a month, the spectacles of the birds that had no carotenoid in the diet or birds that consumed carotenoid-supplemented water with a dog food diet remained dowdy. Carotenoids alone were not sufficient to brighten them up. However, the mynas that consumed carotenoid-supplemented water in addition to selecting how much protein, fat and carbohydrate they ate really rose to the challenge: their eye patches transformed into a vivid yellow. And when the team analysed the birds’ diets, it turned out that males tended to consume the most fat, which contributes to transporting the pigment in the birds’ blood, resulting in the yellowest eye patches. In contrast, the females increased their protein consumption, probably in preparation for reproduction, yet remained drab.
It seems that it takes more to produce vivid skin and plumage than simply consuming the essential pigment. Without a balanced diet that matches their lifestyle, the myna birds cannot capitalise on dazzling carotenoids to give them their essential yellow specs. And the team cautions that other brightly coloured species may be at risk of losing their lustre and allure in cities, where tempting human scraps provide less nutrition. ‘The abundance of anthropogenic food… has the potential to affect color signalling and sexual selection’, Peneaux warns.
