An African bird that uses cuckoos' famous strategy of getting other birds to raise its young has been found to be leveling up. Many cuckoos disguise their eggs to fool unwitting parents, and some young have found ways to deceive when asking for food. However, cuckoo finches have taken to mimicking the looks of common, harmless birds in order to fool other species so that they can lay eggs in their nests.

Cuckoo finches (Anomalospiza imberbis) are neither finch nor cuckoo. However, as a member of the Viduidae they are not too distant from finches. The first half of their name, however, is based on behavior and not on genetics. Like many cuckoo species, they are brood parasites, laying their eggs in other species' nests and leaving those birds to do the hard work of raising their offspring.

Feeding another species' young is a lot of effort for little evolutionary reward. In some cases cuckoo young provide some service for their board and lodging, but most brood parasites simply drain food from the host species and in some cases actively kill their nestmates. Consequently, host species have a powerful evolutionary incentive to not be fooled. This can lead to fascinating contests as each side evolves to gain an advantage.

In the course of his PhD at the Australian National University, Dr William Feeney, now at Cambridge University, studied how cuckoo finches in Zambia trick tawny-flanked prinias (Prinia subflava) into raising their young. The most obvious mechanism is to have eggs of similar size and coloring to those of their hosts. While some cuckoo species have abandoned this approach in favor of straightforward intimidation, egg mimicry is common.

^{The cuckoo finch eggs are on the inside and the prinia eggs are on the outside. Claire Spottiswood.}

However, brood parasites also need to reach a nest unseen to lay their eggs. Feeney noted that female cuckoo finches bear a strong visual similarity to female southern red bishops (Euplectes orix), birds common to the area, which are harmless to prinias. He wondered if this could be because these species live on open savannah where it is hard to sneak into a nest without being seen, unlike the forests favored by true cuckoos. The males, who don't need such strategies, look quite distinct.

Feeney examined cuckoo finch specimens and found that they resemble red bishops far more than other Vidua species to which they are much more closely related. Since the resemblance could not simply be a result of evolutionary history, he reasoned that it must be a disguise.

“This shows that brood parasites use this kind of 'wolf in sheep’s clothing' disguise in all stages of their life cycle: as eggs, chicks, fledglings, and we now know, as adults,” Feeney said.

Inevitably, however, the prinias are fighting back. Feeney reveals in the Proceedings of the Royal Society B that, unable to tell cuckoo finches from red bishops, they have taken to attacking the females of either that get to close to their nests, while leaving the males alone. Moreover, when Feeney displayed either a stuffed cuckoo finch or red bishop around the nest, prinias examined the eggs in their nest more closely and were more likely to spot a cuckoo finch egg.