The Economist again:
The data come from a census of 57 bird species that the researchers carried out in the area affected by the radioactive plume from the accident. When due allowance was made for habitat differences, they found that species which relied on a class of chemicals called carotenoids to tint their feathers fared worse when there was more radioactivity around. Intriguingly, that did not apply to birds that used melanin, another pigment, in their plumage, nor to those that employed iridescence—which is a result of the structure of feathers, rather than their chemistry.
Besides acting as pigments, carotenoids are antioxidants that have an important role in protecting DNA from harm. One of the ways that radiation causes harm is by generating molecules that promote oxidation, so a good supply of carotenoids protects against such damage. Using them to make feathers pretty instead of mopping up oxidative molecules thus has a significant cost—as this result shows.