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GREAT tits that show disgust when they eat a horrible meal may be driving their prey to evolve.

Many prey animals have deterrents against predators, like poison. Some signal this with conspicuous colours.

It鈥檚 unclear how such signals came about. 鈥淧redators need to first learn the signal before they avoid it, but until they do, these prey are easy to detect and kill,鈥� says Rose Thorogood at the University of Cambridge. If predators learn by watching other鈥檚 errors, however, this might protect the prey population long enough for it to reproduce and evolve.

Her team gave great tits a new 鈥減rey鈥�: almonds, some dosed with a chemical they dislike. The almonds were in envelopes, marked if they held a bad one. Some tits watched a bird eat a bad almond. It spat it out, then wiped its beak: a sign of disgust.

All birds learned to avoid the bad almonds through trial and error, but 鈥渢rained鈥� birds ate fewer (Nature Ecology & Evolution, ).

鈥淭he conspicuous 鈥榩rey鈥� were 40 per cent more likely to survive when predators were socially informed,鈥� says Thorogood.

This article appeared in print under the headline 鈥淕rossed-out great tits steer evolution鈥�