Carotenoids are bright red pigments and they are usually only found in plants and fungi, not animals. Animals will use carotenoids, often to display their fitness to potential mates, however. Take the cardinal. The male cardinal's color comes from his diet. He has to work hard to obtain foods rich in carotenoids to make his feathers brighter than all the other male cardinals around so when it comes time for the females to pick a mate they will choose him. It's a sign that he's healthy and strong and able to spend a lot of time foraging. So he'll be able to be a good dad and help out when the babies come. Plus he has strong genes and every mom wants their children to have strong genes.
I could only find the abstract for the paper and I'm really tempted to pay for the full article because it leaves some big mysteries. The abstract states that the red color "influences their susceptibility to natural enemies," but I'm wondering if that means birds and other animals are more likely to eat them since they contain coratenoids? Is there some advantage that the abstract does not state? What's the selective advantage? And also how the heck did those fungus genes get in there? Parasitic red fungi? I know that our genome is littered with genes from other things but those are mostly from viruses which makes sense.
So if you have any more knowledge or if you're somebody with maybe a background in genetics and have a better understanding of this stuff, please, drop me a line.