The relationship between cuckoos and their hosts is a classic example ofan evolutionary arms race. Cuckoos, should evolve eggs that more closely match those of their hosts, while the hosts should evolve keener senses to discriminate between their own eggs and a cuckoo’s.

But in Africa, this classic story takes an unusual twist.

The greater honeyguide isn’t a cuckoo but uses the same tactics—it parasitises the nests of little bee-eaters by laying eggs of the same size and shape. But this mimicry doesn’t help it to fool the bee-eaters, which seem to accept any old egg no matter how different it looks. Instead, Claire Spottiswoode from the University of Cambridge has found that the parasitic honeyguides are fighting an evolutionary arms race against… each other.