As winter marches towards spring, two male northern cardinals have set up territories in our yard, keeping an eye on their mates but not getting along with each other.  They watch, emblazoned against a blue sky, from a favorite perch in bare trees. Catching sight of each other they ‘see red’, puffing up scarlet robes which get more brilliant as the breeding season approaches.

Why does red stand above all others, sometimes a sign of menace, sometimes a symbol of courage, and often expressing the power of joy and passion?

Red brake lights, red stop lights, red London buses, red light district, red planet, Redcoats, Red Army, red flags, cardinal bishops, Coca-Cola … the list goes on.  A color so intense it seems to sear the back our brain, where circuits originating in the red cone receptors of the retina are processed in the visual cortex. Would red convey a smaller emotional charge if blood was blue or fire green?

There’s nothing in a color beyond a wavelength – apart from how the brain perceives it. Hot peppers come in green as well as red, and red letter days and red herrings are fairly benign! Most mammals lack the red cone, seeing gray instead, but they are capable of as much passion as any primate or bird that has full color vision.  Bulls charge at a waving ‘gray’ flag.

To ask how a bird gets as hot as a cardinal is like asking for a Just So story – How the leopard got his spots … How the camel got his hump? Well, it’s the diet, dum-dum, because everyone knows, “We are what we eat.”  And looking around the yard, there is ostensible evidence everywhere.  Holly and Nandina bushes tempt hungry birds with dangling crimson berries.  But wait, biologists seldom answer a simple question with a straightforward story. Most of the intensity in these berries is from anthocyanin pigments which are not absorbed intact into the bloodstream. If berries are not wholly responsible, perhaps birds get their color from something else in their diet – like insects.  Errh … not the cactus juice-sucking insects that gave us cochineal for the pantry and for dyeing the tunics of Redcoat officers.

Most old sayings become threadbare in the light of research, and with a few exceptions there is no simple relationship between what we eat and how we look because of the rumbling process of metabolism going on inside.  Otherwise some of us would look like … well, I leave that to your imagination.

What then lends cardinals their color – and to so many wonders in nature, from fall leaves to appetizing vegetables?  Bugs Bunny would say that carrots are a good place to begin nibbling:

“Oh, carrots are divine, you get a dozen for a dime …”

Nearly two hundred years ago, a German student, one Heinrich Wickenroder, extracted from carrot juice a “yellow fatty oil with carotin” which dissolved in ether but not in water.  He won praise from his professors who thought it might fend off gut parasites, which were rampant in those days and still all too common in domesticated animals and wildlife.  Over time, his discovery expanded into a family of related molecules from plants, called carotenoids, including lycopene, lutein, and beta-carotene, which is the precursor of vitamin A. If I was asked to score my favorite molecules, carotenoids would be in the top ten beside DNA and chlorophyll.

Carotenoids are needed for photosynthesis, so they are almost ubiquitous in the plant kingdom, from whence we satisfy our own needs.  They prevent blindness, promote fertility, inhibit cancer, block oxidation, boost immunity, and make dandies of our avian friends in the yard.  I love the biological symmetry: carotenoids make plumage conspicuous in birds and give the power of vision so their mates and rivals recognize it and we can enjoy it (in fact, vitamin A is only responsible for the rod cells). Because we can’t make these virtuous molecules in our bodies but have to absorb them from our diet, the food police keep heaping more fruit and veg on our plates.

Northern cardinals obtain their supply from an omnivorous diet of berries, seeds, and insects, while American goldfinches subsist on strictly vegetarian fare.  When Cornell ornithologists reduced red carotenoids in the diet, cardinals grew pale, and goldfinches turned orange when yellow carotenoids were deficient. But changing diets didn’t go as far as switching their colors because the gut is wise to the vagaries of nature (and researchers) by converting one type of molecule to another.  It’s obvious that diet alone can’t explain everything because female cardinals eating the same food are so much drabber than males.  It takes testosterone to make a male hot.

Red grouse are hardly that.  A game bird looking like a large cardinal in the open heather would soon end in a vixen’s den or on a Scottish table. But the males have a small red comb, a token that performs the same service as the cardinal’s feathers.  Since grouse are plagued by roundworms and ticks, ornithologists wondered whether treating the males with anti-helminthic drugs could improve their breeding performance as well as their overall health. They did.  After ridding them of parasites, the birds absorbed more vitamins from their food which, in turn, increased the color intensity of their combs.  The little red flag on their heads was more attractive to the hens, who thought they would make healthier partners.  Female cardinals get the same message. They can’t be hoodwinked by a frail male because carotenoids painting the breasts of males bursting with blushing pride make honest birds of them.

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