“Blue tits are really ultraviolet tits”

It’s a classic stoner conundrum: “We all say the grass is green, right? But what if, like, what you see as green is what I see as blue?”

It’s weird to think that two people could perceive colors in totally different ways without realizing it. But forget people–when it comes to different color perception, we should be talking to birds.

I’m reading Sean B. Carroll’s The Making of the Fittest, and I’ve learned that birds see color differently than people. It all boils down to opsins: light-sensitive proteins in the photoreceptor cells of our eyes. Humans have three kinds of opsins, one each for short, medium, and long wavelengths. The short-wave sensitive opsin is what determines the upper limit of the our color vision. For humans, this opsin is tuned to 417 nm–that is, violet. Some birds have their short-wave sensitive opsin tuned higher–around 370 nm, which means they can see into the ultraviolet range.

This has important implications for bird behavior. Starling females choose mates based on UV-reflecting feathers on their throats. Researchers studying these birds filtered out UV light and the females’ preference was thrown off. Like starlings, male blue tits have UV-reflecting plumage–theirs is on the crest of their heads. This discovery prompted University of Bristol researchers to declare that “blue tits are really ultraviolet tits”.

Ultraviolet vision also plays a role when these birds raise a family. The mouths of chicks from eight different bird species have been shown to reflect UV. Hungry chicks flash these ultraviolet beacons to their worm-providing parents. The strongest chicks have the most reflective beaks.