My second paper of the year and 5th total. This time, back to my roots, birds, with a meta-analysis that shows carotenoid-based feather coloration is an honest signal of some, but not all measures of quality. For birds to display yellow or red feather coloration, they must first eat food that contains pigments called carotenoids. But birds mostly eat yellow carotenoids. To produce red feathers, birds must first convert the yellow pigments to red. The importance of this conversion step in linking color to quality has not previously been investigated.
Here is a link to the paper
We found that, after controlling for several potentially confounding factors, it looks like carotenoid conversion helps to make the link between feather coloration and individual quality stronger. These results should help future studies to focus on the specific mechanisms that link carotenoid conversion to a strong signal of individual quality in birds and other animals.