Sometimes animal coloration has an obvious adaptive explanation. Snowshoe hares are white in winter because individuals that blended with the snow were less likely to get eaten than those that didn’t. A kingsnake that looks like a venomous coral snake is less likely to get preyed upon than one that is only a poor mimic of it. And a gray treefrog that can change its coloration in seconds is better off than one with a permanent lichen-like patterning affixed to its skin.

But how can we explain such brilliant redness as in the male northern cardinal? Simple functional explanations can be immediately discarded. We do not live on a lava flow, the bird would gain no benefit from being the color of mammalian blood, and natural selection does not operate according to what humans find pretty.

So yes –– why look like a Christmas ornament? The assumption that it pays more to be cryptic than conspicuous is a good one in many cases. But the principle fails to explain the peacock’s tail, the woodcock’s mating dance, the wren’s virtuosic dawn jubilee, and the mandrill’s face. What we see in the male Cardinal is a signal of mate quality, a product of sexual selection.

Though all male cardinals appear to have brilliant red plumage to us, there seems to be variation in the trait. And redder male cardinals tend to be larger, in better condition, and to invest more into parental care. Redness is also not the only sexual ornament in male cardinals. Cardinals with smaller black face masks are more parasite resistant, as are cardinals with redder bills.

These characteristics serve as heuristics, “rules of thumb,” which females use in mate choice. The decision to mate is a costly one for females. Visual signals, like red carotenoids, are costly to produce, which makes them honest.

Over time, females that recognized these signals accurately left behind more of their genes than females that didn’t. So the next time you wonder why the male Cardinal’s plumage defies all laws of survival, remember it as an enticing mating cue.