SONGBIRDS use a private communications channel to attract a mate without being spotted by predators.
Birds’ eyes have four colour receptors, one more than mammals. In songbirds the extra receptor is sensitive to ultraviolet light. But the receptor in hawks and crows is most sensitive to the violet end of the visible light range. The difference is larger than that between the green and red colour receptors in humans, says Anders Ödeen of Uppsala University in Sweden.
Ödeen’s group measured the UV brightness of sexual-signalling “badges” on the head and chest of 18 songbirds, including the goldcrest, willow tit and long-tailed tit. They say that 80…
