Why Are Cardinals Red? Diet, Pigments and Choosing Mates

Carotenoids: color from food

Carotenoids are natural pigments made by plants, algae and some fungi and bacteria. They give carrots, tomatoes, autumn leaves and egg yolks their color. Birds cannot make carotenoids themselves, so every bit of red, orange or yellow carotenoid color in their feathers starts in their diet.

The northern cardinal eats mostly seeds and fruit, such as wild grapes, dogwood berries, sumac and weed seeds, plus insects in summer. These foods contain mostly yellow carotenoids. When the bird is growing new feathers, its body carries the pigments in the blood to the feather follicles, where they become locked into the growing feather.

How cardinals turn yellow pigment red

The male cardinal’s crimson is not simply copied from its food. Much of it comes from red pigments called ketocarotenoids, which the bird makes by chemically modifying yellow carotenoids. Experiments on captive cardinals showed this clearly: males fed only yellow carotenoids still turned red, although a paler red than wild birds.

In 2016, two research teams studying zebra finches and red canaries identified a gene, called CYP2J19, that produces an enzyme involved in converting yellow carotenoids into red ketocarotenoids. The same gene also helps produce red oil droplets in birds’ eyes that aid color vision. Its discovery helped explain how many red birds, cardinals included, can create red from yellow food.

Why color is set at molt

A feather is dead tissue once it has finished growing, just like human hair. That means a cardinal’s color is fixed when the feather grows. Adult cardinals replace their feathers once a year, in a molt from late summer to fall.

A male that eats well and stays healthy during the molt grows bright red feathers. One that is short of food, carrying parasites or sick at that time may molt into duller, more orange plumage and stay that way until the next molt. Fresh fall feathers also have slightly grayish tips, which wear off over the winter, so males actually look brightest in spring.

Why do brighter males do better?

Because red color depends on diet and health, it works as an honest signal of quality. A bright male is showing that he found plenty of food and stayed fit while growing his feathers.

A study of cardinals in the 1990s found that redder males tended to hold territories with denser vegetation, which offer better cover for nests, and had greater breeding success. Females may also use plumage color, along with song and courtship feeding, when choosing a mate. Rival males pay attention too: color helps signal how strong a competitor is.

Brightness alone is not the full story, however. Song, territory quality, age and experience also influence which males breed most successfully.

Why are female cardinals duller?

Female cardinals are mostly warm buff-brown with red tinges on the crest, wings and tail and a bright red-orange bill. The difference between males and females, called sexual dimorphism, is common in songbirds.

Two pressures shape it. The female does all of the incubating, sitting on the nest for 12–13 days at a time, so plumage that blends with leaves and twigs helps her avoid predators. Males, meanwhile, are under stronger pressure to impress females and rivals, which favors bright color. Females still carry some red, and recent research suggests female color can also signal quality. To compare the sexes, see what a female cardinal looks like.

Rare yellow cardinals

Every few years, a bright yellow cardinal appears at a backyard feeder and makes the news. These birds are northern cardinals with a rare genetic variation that prevents them from converting yellow carotenoids into red. The black face mask stays black, but the feathers that would normally be red are yellow or yellow-orange.

Yellow cardinals are not a separate species or a hybrid. They behave like normal cardinals, and some have been seen paired with normal red mates.

Cardinals compared with goldfinches and blue jays

Other backyard birds use the same chemistry in different ways. The male American goldfinch is bright yellow in summer because it deposits yellow carotenoids from seeds in its feathers without converting them to red. Unlike the cardinal, the goldfinch molts twice a year and turns olive-brown in winter.

Blue birds work very differently. There is no blue pigment in a blue jay’s feathers: the blue comes from the microscopic structure of the feather, which scatters blue light. Read are blue jays really blue? to learn how.

The cardinal’s name and color

The northern cardinal was named after the cardinals of the Roman Catholic Church, whose robes and caps are a similar scarlet. Its red has made it one of the most recognizable birds in North America and a favorite on winter cards, where it stands out vividly against the snow.