Seeing More Colours Than We Do

[A] Human colour vision depends on three types of cone cell in the retina, each sensitive to a different part of the spectrum. Most birds have four. [B] The extra cone responds to ultraviolet light, which lies beyond the range our eyes register at all. This is not a small difference of degree. A visual system with four channels can distinguish combinations of wavelengths that a three-channel system reports as identical.

[C] The consequences show up in places that once puzzled ornithologists, and they are not confined to a handful of exotic species. Dozens of birds in which males and females appear indistinguishable to us turn out to differ sharply under ultraviolet light, which removes the need to explain how such birds recognise a mate. [D] Berries that look uniformly dark to a human eye carry ultraviolet markings that advertise ripeness. Even the plainest plumage may be carrying a signal, invisible to the observer holding the binoculars but perfectly legible to the animal it is addressed to. What we call drab is a statement about our own retinas, not about the bird.