The Northern Cardinal shows a notable difference in appearance between the sexes. Males display vibrant red plumage, while females exhibit a more subdued, pale brown coloration, often with reddish tinges on their wings, tail, and crest. This distinct contrast prompts curiosity about the biological reasons behind the female’s less conspicuous coloring.
The Biology of Bird Coloration
Bird coloration arises primarily from two types of pigments: carotenoids and melanins. Carotenoids, responsible for red, orange, and yellow hues, cannot be produced by birds. Instead, birds acquire these pigments through their diet, by consuming plants or other organisms that have ingested plants containing carotenoids. Male cardinals obtain their vibrant red color by converting yellow carotenoids from their diet into red pigments within their bodies, a process aided by a specific enzyme.
Melanins are synthesized by birds’ bodies, producing colors like blacks, grays, and browns. There are two main forms of melanin: eumelanin, which results in dark black, gray, or brown, and pheomelanin, which produces lighter yellowish to reddish-brown shades. Female cardinals primarily display brown coloration due to melanin pigments in their feathers. This difference in pigment reliance contributes to the color divergence observed between male and female Northern Cardinals.
Evolutionary Purpose of Female Brown Plumage
The brown plumage of female cardinals serves a crucial evolutionary purpose, primarily providing camouflage. This subdued coloration allows females to blend seamlessly into their surroundings, such as dense shrubs and trees, particularly when nesting or incubating eggs. Remaining inconspicuous helps protect the female, her vulnerable eggs, and young chicks from predators.
While the male’s bright red plumage is thought to attract mates and signal health, the female’s less flashy appearance is directly linked to survival and reproductive success. Natural selection has favored females with cryptic coloration, as those blending into the environment are more likely to successfully raise offspring. This difference highlights a division of roles, where the male’s color is for display and the female’s is for protection and nurturing.
Variations in Female Plumage
Female cardinals are not uniformly brown; their plumage can display subtle variations, often including hints of red. These reddish tinges are typically found on their wings, crest, or tail. Several factors contribute to these individual differences in coloration.
Age can play a role, with older females sometimes exhibiting more red highlights than younger ones. Diet is another important factor, as the availability and intake of carotenoid-rich foods can influence the extent of reddish coloration. Genetic differences among individual birds also account for some variation. Despite these subtle differences, the predominant brown coloration remains a consistent and advantageous characteristic for female cardinals.