Since infancy, our world has been like a vibrant painting, mixed from a palette of optic primary colours: red, blue and green. But for birds, the planet is a much more lurid place. Our feathery friends cannot only see our spectrum but beyond into the UV. Colour photoreceptors in the retina called cones are stimulated by light. In Man, the relative activity of three cone types measures the amount of red, green and blue wavelengths, and determines which `colours' we see. The combined output of these cones measures total brightness. Some birds like quail have a fourth blue cone with sensitivity into the UV, whereas others like starlings posses a fourth cone for UV alone. It has long been known that birds can detect UV, but no one was sure whether birds see it as colours or as increased brightness. Emma Smith, Verity Greenwood and Andrew Bennett set out to...

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