I have watched my fair share of bird documentaries. Almost all include courtship and mate selection rituals. These are often bizarre or weird to our eyes. That is until we think back to our teens and the corollaries come racing back to mind.
One of my unfair conclusions early on was that the females (it is most often the females making the final selection decisions, but not always) were making a shallow ‘pretty boy’ selection. A kind of avian version of the Bachelorette not-so-real reality television show. I discovered when researching structural color (for this post) versus pigment coloration, that feather color saturation and brightness can be an ‘honest’ signal of an individual specimen’s health.
This appears to be the case in particular for species that rely on carotenoids for yellow, orange, or red pigments. Carotenoids are provitamins and antioxidants and have been linked to improved immune system function, bird and human. The thinking concerning birds is that an individual that consumes a healthy diet can use the carotenoid intake in excess of what’s needed for a healthy immune system for more feather pigments and thus brighter and healthier looking feathers. This really turns the females on and gets a male an honest A on his health report card.
A species that demonstrates this really well is American Goldfinch. They are 90%+ vegetarian. Any insect protein is incidental. A healthy, carotenoid-rich diet and the high functioning immune system it brings to the table is in plain sight in their breeding plumage. Yes, the male notices the female’s health and fitness too.
Hooded Warblers, on the other hand, are almost exclusively insectivores. Where to they get their carotenoids? By eating arthropods that eat plant material and bring the catenoids along. For example, some caterpillars are an unusually rich source of carotenoids. A primarily yellow\olive colored warbler that depends on the colors for camouflage in the canopy for survival requires a healthy supply of carotenoid-rich arthropod diet. Individuals that are really good establishing territories rich in arthropods and are successful foragers (eating the wrong caterpillars can be hazardous to one’s health) will have the excess carotenoid pigments to use for appropriately showy brilliance during breeding season. An honest signal of health for the discriminating mate.
This is why one of the most important conservation actions we can take is to make any and all property we own, or can influence decisions about, is to stop indiscriminate killing of insects. Even better, is to intentionally make it attractive to insects. Native plants and trees, especially oaks, are arthropod heaven.