The purple patch

To the human eye, the sexes of the blue titParus caeruleusare virtually indistinguishable. The results of two different groups now show that there are in fact differences in the colouring of male and female tits, but that they fall in the ultraviolet part of the colour spectrum — which the birds can see but we and other mammals cannot. Why this signalling system should have arisen and endured poses some thought-provoking questions for evolutionary biologists.