Everyone likes to look good, and greenfinches are no exception. They tell the world how fit and well they are through the brightness of their plumage. Peeter Hõrak from Tartu University, Estonia, explains that the carotenoids – which produce vivid yellow, orange and red pigments – were believed to function as antioxidants protecting birds from the damaging effects of oxidation: the brightest birds were thought to be the strongest because of their resistance to oxidative damage. Dark black feathers are also a sign of fitness, but black melanin pigments can only be produced if the levels of a powerful antioxidant, glutathione, are low, leaving the bird vulnerable to oxidative damage. So being fit with bright yellow feathers and maintaining strong antioxidant defences would seem to be incompatible with simultaneously having healthy black feathers, where the bird's antioxidant defences are low. But greenfinches seem to handle this contradiction fine; they have yellow and black tail feathers. Intrigued by the contradiction, Hõrak and his colleagues decided to find out what messages birds' colours send ().
Collecting wild greenfinches from around Tartu, Hõrak and his students Elin Sild and Tuul Sepp brought them back to the lab. Supplementing the diets of some of the birds with carotenoid spiked water and reducing the glutathione levels of others with injections of buthionine sulfoximine, the team plucked one yellow and black tail feather from each bird and then waited for the feather to grow back to see what effect the treatments had on their colours.
Monitoring the birds' health while waiting for the feathers to grow, the trio also teamed up with two medical biochemists, Kalle Kilk and Ursel Soomets, to measure the greenfinches' levels of malondialdehyde, which shows the amount of oxidative damage sustained by the animal. ‘Kilk and Soomets have a good and reliable assay for measurement of malondialdehyde based on HPLC and mass spectrometry that we used to show whether there is a clear link between oxidative damage, glutathione and black coloration,’ explains Hõrak.
Measuring the darkness of the tips of the newly grown tail feathers, the team saw that the birds that had received glutathione reduction injections had low glutathione levels, the darkest feathers and suffered the highest levels of oxidative damage. Hõrak explains that the greenfinches' black tail feather tips were showing that the birds were fit because they were tough enough to cope with disease: like drinkers that consume too much alcohol and brag about ‘being able to take it’. But what effect did the carotenoid supplemented water have on the birds' health and plumage?
Sure enough, the yellow portion of the new tail feathers grown by the greenfinches on a carotenoid supplemented diet were brighter than the tail feathers of birds that did not have additional carotenoid. However, the carotenoids did not protect the birds from oxidative damage. Kilk and Soomets' measurements showed that the birds' malondialdehyde levels were as high as those of the birds who did not receive additional carotenoid. The carotenoid boost had not protected them from oxidative damage.
So bright yellow feathers do not signal the birds' resistance to oxidative damage, which answers Hõrak's initial question: there is no contradiction. Birds can have bright yellow and dark black feathers at the same time because the colours send different messages. While black pigments are attractive because they signal that a bird has good defences and is tough, it is less clear what bright yellow plumage signifies; but whatever it is, the lady birds love it.
