With their distinctive green-yellow plumage, greenfinch males are not bashful when boasting about their fitness. Elin Sild and colleagues from Tartu University, Estonia, explain that the bird's vivid colour is produced by antioxidant carotenoid pigments, which can only be incorporated into their plumage if not required for other purposes – such as reducing the levels of damaging reactive oxygen species (ROS) circulating in their blood. However, the team explains that this simple relationship has recently come under debate, as the birds' phagocytic cells (part of the immune system) also produce ROS to combat bacterial infections. In this case, the high carotenoid levels indicate by bright plumage could counteract the immune system. Evidence supporting both sides of the argument has been accumulating so, the team decided to test how carotenoids in the birds' diet affected ROS levels in the birds' blood (p. 3467).
Collecting adult male birds from a garden in Tartu, the team supplemented the birds' diets with two carotenoids, monitored their blood carotenoid levels and found that the carotenoid levels increased by 40–53% compared with birds that had not received the supplement. Next, knowing that the bird's immune systems produce ROS in response to bacteria infections, the team tested the effects of a simulated infection on the amount of carotenoids in the birds' blood and on their ROS levels. Injecting the birds with lipopolysaccharide (a component of bacterial cell walls), the team found that the bird's carotenoid levels fell. However, when they injected the birds with dead Brucella abortus bacteria and measured the amount of ROS produced by the birds' phagocytic cells in response to the fake infection, the team found no change in ROS production by the phagocytic cells. So the carotenoids did not counteract the production of ROS by the birds' immune systems, suggesting that carotenoids do not provide the link between bright plumage and ROS production during the immune response in greenfinches.