The proximate (immediate) answer to the question of why cardinals are red is that they have a diet rich in carotenoid pigments (e.g., the pigment that makes carrots orange), and will incorporate those pigments into their feathers as they develop each year during feather molt. To read more about this process, click here to read an article from Bird Watching Daily that explains where feather colors come from.
But I’m interested in the evolutionary basis for why cardinals stand out among other species residing year-round in northern climates in retaining their bright red coloration instead of molting to more muted shades during the non-breeding season, like, for example American Goldfinches do. Cardinals undergo a post-breeding feather molt, just like other species, so why risk attracting predators with bright red, when you might escape notice with a dull brown plumage instead?
Does sexual selection (that is, female choice for the most beautiful male, as Richard Prum argues) play a role? Please see the yesterday’s Back Yard Biology post for more information about the importance of beauty in female choice.)
Some additional facts regarding male-female relationships in Cardinals and their close relatives, the Pyrrhuloxias of the southwestern U.S., might help to answer the question.
The first thing you notice from the illustration above is that the Pyrrhuloxia male and female look more alike than Northern Cardinal male and his mate do. Why might that difference have arisen in such closely related species? And does it have anything to do with female choice for brightly colored, beautiful males?
The second and interesting difference between the two species is that Pyrrhuloxia share a pair bond only during the breeding season, breaking up in the non-breeding season into loose, often mixed-species flocks of seed-eating birds that move around to find food in their dry, desert habitat. Each year, males vie for available females by staking out and advertising their territories, often very aggressively. In contrast, Northern Cardinals may maintain their pair bond year-round, often for multiple years, unless one of the pair dies, and the pair stays in the same general area in which they nested, year-round.
Female Cardinals are making a choice of more than just a breeding partner; they may be choosing a life partner and his residence (territory). Richard Prum might argue: what’s a male to do, but be the most beautiful thing she has ever seen? And if he is better looking than his neighbors, she stays with him through that year and the next. They raise multiple broods of young Cardinals together, and he passes on his good looks to the next generations.
This is an amazing high definition video of male and female Cardinals feeding their chicks (by FrontYardVideo).
In other words, it’s possible that their long-term pair bond and high fidelity to a territory year-round makes it possible for choosy female Cardinals to drive the accentuation of beauty (in this case, “redness”) in their male partners, to the extreme that male Cardinals never lose their red color during the year.