There's an arms race going on in the birds' nests of Africa, an on-going evolutionary fight between cuckoos and the birds whose parenting skills they steal.

Cuckoos lay their eggs in other birds nests', which the unwitting parents then raise as their own. But researchers at the University of Cambridge in the United Kingdom have found that the parasitized bird species are fighting back, by out-evolving the cuckoos.

One bird, the tawny-flanked prinia, has taken to laying eggs with wildly varying colors and patterns, making them harder for the cuckoo finch to copy. When the prinia parent finds a cuckoo finch egg in its nest, it spears the egg with its beak and carries it away.

"These variations seem to act like the complicated markings on a banknote: complex colors and patterns act to make host eggs more difficult to forge by the parasite, just as watermarks act to make banknotes more difficult to forge by counterfeiters," Claire Spottiswoode, a fellow at the University of Cambridge's Department of Zoology, said in a release.

Another bird, the red-faced cisticola, has evolved a different strategy. Its eggs are only moderately variable, but it's become "extremely discriminating" in its ability to decide whether an egg is from a cuckoo or a cisticola and can now spot even a sophisticated mimic.

Another, the rattling cisticola, has done such a good job of both creating complicated and varied eggs and learning to recognize cuckoo eggs that the cuckoos have given up, its eggs are no longer parasitized.

The paper is in this week's edition of the journal Proceedings of the Royal Society B: Biological Sciences.