The bright red plumage of the male Northern Cardinal is one of the most recognizable features of this common backyard bird. The females, on the other hand, have plumage that is tan, gray, or olive-colored. This distinct sexual dimorphism, where males and females of the same species display different coloration, is quite common among North American bird species. But why is it only the male Northern Cardinals that sport the vivid red feathers? The reasons behind this are related to evolution, sexual selection, and the role of hormones like testosterone.
The Role of Testosterone
Research has shown that the red coloration in male Northern Cardinals is directly linked to testosterone levels. Testosterone stimulates the production of carotenoid pigments, which are responsible for the bright red hues. The more testosterone a male cardinal has, the redder its feathers will be. Females have much lower levels of testosterone, insufficient to trigger full carotenoid production.
During breeding season, male cardinals’ testosterone levels are high, resulting in the brightest red coloration to attract females. In the non-breeding season, their testosterone levels drop, leading to a duller shade of red. The seasonal color changes are much subtler in females since their testosterone levels are consistently low.
Carotenoid Pigments
Carotenoids are organic pigments produced by plants and acquired by animals through their diets. In birds, carotenoids are responsible for red, orange, and yellow plumage. Animals do not naturally produce carotenoids, so they must obtain these pigments by consuming carotenoid-rich foods like seeds, fruits, insects, and crustaceans. The carotenoids are then deposited into new feathers as they grow.
Cardinals get red carotenoids like cannothin, lutein, and zeaxanthin from wild berries, seeds, and insects. The more carotenoids circulating in the bloodstream, the richer the red coloration of new feathers. Male cardinals, with their high testosterone, are primed to produce the most vibrantly carotenoid-infused plumage.
Evolutionary Advantage
From an evolutionary perspective, the male cardinal’s bright red coloration provides an adaptive advantage. The bold color helps males stand out to potential mates and assert their dominance over competitors. Researchers have found that the reddest males tend to be healthier, hold better territories, and have greater reproductive success. So over time, natural selection has favored this coloration in males.
Additionally, the red plumage enables males to be easily spotted by females against winter backgrounds. This helps facilitate pairing up during the breeding season. The more subtle, camouflaged female plumage, on the other hand, allows females to blend into their surroundings while incubating eggs.
Sexual Dimorphism
Sexual dimorphism, where males and females of the same species display distinct physical differences, is common in the animal kingdom. It arises when evolutionary pressures differ for each sex. In many bird species, males have evolved showy plumage for courtship while females have more cryptic colors for nesting.
Cardinals exemplify this pattern. Males need bright colors to attract mates, so testosterone-induced carotenoid deposition provides an advantage. Females don’t participate in courtship displays, so there’s no evolutionary benefit to them producing costly red pigments. Their drab colors help conceal the nest.
This divergence suggests the sexes are adapting to their contrasting reproductive duties. Over time, these adaptations lead to sexual dimorphisms like that seen in cardinals.
Hormone Regulation of Coloration
Researchers have definitively linked male cardinals’ red hues to regulated carotenoid production under the influence of testosterone. During the breeding season, surging testosterone drives the creation of the reddest plumage. Experiments where male cardinals were castrated resulted in a complete loss of red color as testosterone levels crashed.
Injecting testosterone back into castrated males caused them to regain full red pigmentation. Interestingly, when female cardinals were given testosterone supplements, they too began displaying red plumage. The hormones were stimulating carotenoid production in both sexes.
These studies clearly demonstrate how testosterone acts as the switch controlling carotenoid conversion and deposition in feathers. Male cardinals are essentially programmed to maximize redness each breeding season in response to their naturally high testosterone.
The Importance of Diet
While testosterone drives carotenoid production, cardinals must get these pigments through their diet. Cardinals obtain carotenoids from eating certain seeds, fruits, insects, and crustaceans. Common sources include wild berries, sumac seeds, dogwood fruits, and aphids. Without consuming enough carotenoid-rich foods, even males will fail to achieve full red coloration.
In backyard settings, cardinals can get supplemental carotenoids at bird feeders. Sunflower seeds, safflower seeds, and fruit slices all provide carotenoids. Optimal plumage redness requires a balance of both hormonal control and proper nutrition.
| Carotenoid | Food Sources |
|---|---|
| Lutein | Kale, spinach, corn, eggs |
| Zeaxanthin | Corn, orange peppers, oranges, salmon |
| Astaxanthin | Salmon, trout, krill, algae |
| Capsanthin | Red peppers, paprika, pumpkin, sweet potato |
Geographic Variation
Interestingly, not all male Northern Cardinals have the same red hue. Their exact shade can vary geographically across populations. Northern cardinals in the subtropics tend to be rich crimson, while those in the southwest are more orange-red. The pigments they ingest from local diets contribute to these subtle differences.
There are also regional gradations in female coloration. Northeastern females are olive-brown, Great Plains females are grayish-brown, and southwestern females have a reddish-brown wash. Overall though, males are distinctly redder across all populations. Climate, diet, and testosterone levels all intersect to determine precise shades.
Close Cardinal Relatives
The Northern Cardinal belongs to the family Cardinalidae, which includes grosbeaks, saltators, and other New World songbirds. Many close cardinal relatives share a similar sexual dimorphism. Male scarlet tanagers, vermilion flycatchers, and summer tanagers glow red against their female counterparts. Yet certain cardinals, like the Yellow-billed Cardinal, lack red completely. Again, specific carotenoids in local foods, plus evolutionary pressures, shape male coloration.
But only the Northern Cardinal, with its year-round residence across North America, provides such reliable opportunities to spot a brilliant red bird against the landscape. For many people, the male cardinal is the first red songbird they recognize.
Cardinal Coloration Changes Over Time
While Northern Cardinals have likely always displayed sexual dichromatism, research suggests male coloration has intensified over time through sexual selection. Early in cardinal evolutionary history, males may have been only slightly redder than females.
But because the reddest males obtained more mates and sired more offspring, subsequent generations saw this coloration spread. Today’s male cardinals now exhibit maximum redness. Interestingly, other North American songbirds, like scarlet tanagers, may now be undergoing similar intensification.
So while male cardinals have always been redder than females, their perfect hue today arose gradually over many generations. The same evolutionary drivers continue to shape cardinal colors.
Molecular Basis of Red Coloration
The molecular mechanisms behind cardinal pigmentation are complex. Carotenoids themselves absorb blue-green light and reflect back red and yellow wavelengths, giving rise to macro coloration. But feather structures also scatter and intensify reflected light through nanostructures.
Cardinal feathers contain hollow channels called beta-keratin rods. Air spaces in these channels scatter light to amplify carotenoid colors. Males maximize these structures, along with lipid absorbance layers, to optimize brilliance. The molecular architecture of feathers is itself selected to accentuate redness.
So color depends on an interplay between pigments, feather keratin proteins, air spacing, and lipid layers. Slight alterations to any of these can tweak the final vibrant plumage. Cardinals have thus evolved precise molecular mechanisms in both pigments and feathers to perfect their signature red.
Condition-Dependency of Ornaments
A male cardinal’s redness provides honest information about its fitness. Healthier, higher quality males produce richer, more vibrant plumage. This is because carotenoids have antioxidant properties that boost immune function.
Only the strongest males can spare enough carotenoids for coloration instead of using them for self-maintenance. And testosterone itself has immunosuppressive effects, so only the most robust males can handle high testosterone levels without succumbing to parasites.
Thus, red plumage indicates a male has both a strong immune system and good foraging skills to find carotenoids. Females can use color as a reliable fitness gauge. This condition-dependency perpetuates the value of the red signal.
Future Threats to Coloration
While Northern Cardinal coloration remains vivid today, pressures from urbanization, pollution, and climate change could someday impact pigmentation. Lower carotenoid availability, endocrine disruption, and weakened health may inhibit full redness.
However, cardinals readily adapt to backyard habitats and supplement wild foods with feeders. This plasticity should enable cardinals to maintain healthy coloration despite environmental changes. As long as males can access the carotenoid-rich diet needed to fuel their testosterone-regulated displays, their crimson hues will likely persist.
Significance of Cardinals and Red Feathers
Northern Cardinals are so symbolic that they represent the states of Illinois, Indiana, Kentucky, North Carolina, Ohio, Virginia, and West Virginia. Their red plumage has long captivated people.
Cardinals also provide convenient opportunities to study evolutionary principles, avian physiology, sexual selection, and carotenoid biochemistry. Analyses of cardinal coloration have contributed greatly to our understanding of birds.
By investigating the mechanisms behind the male cardinal’s trademark red feathers, scientists have gained broader insights into avian coloration and its relationship with diet, hormones, health, and the evolution of dimorphic traits. These photogenic birds continue to be prime research subjects.
Conclusion
Male Northern Cardinals owe their brilliant crimson plumage to evolutionary drivers, specialized feather structure, high testosterone levels, and diets rich in carotenoids. Cardinals likely evolved this sexual dimorphism because bold red coloration gave males an advantage securing mates and territories. Over many generations, evolution amplified male redness while maintaining female camouflage.
The complex interplay between hormones, pigments, feather proteins, immune function, and nutrition that generates the male cardinal’s color makes him an intriguing case study. By understanding how cardinals produce different hues, we elucidate general principles of animal coloration, communication, sexual selection, and adaptation. The male cardinal’s beauty offers a window into the dynamic evolutionary forces that shape morphology and behavior.
