Move over cuckoos, there’s a new bird in the nest. While common cuckoos are masters of egg mimicry, laying eggs in the nests of other bird species to fool them into rearing cuckoo chicks, research has revealed that a group of parasitic finch species in Africa has evolved to take the art of copying a step further.

Working in the savannahs of Zambia, a team of international researchers collected images, sounds and videos over four years, focusing on a group of fi nches occurring across much of Africa called indigobirds and whydahs (collectively known as vidua finches).

The researchers found that, just like cuckoos, the 19 different species within this group lay their eggs in the nests of other birds in an attempt to fool unwitting parents into rearing them as their own. But with these finches, the subterfuge continues even when the chicks are born. Each species of indigobird and whydah chooses to lay its eggs in the nests of a particular species of grassfinch. Nestlings of the parastic vidua finches were found to mimic the appearance, sounds and movements of the grasfinch chicks, right down to the same elaborately colourful patterns on the inside of their mouths. 

The pin-tailed whydah is a remarkable bird, both as an adult, and as a parasitic chick

The mimicry is even more remarkable because grassfinches are unusual in having brightly coloured and distinctively patterned nestlings, and nestlings of different grassfinch species have their own unique appearance, begging calls and begging movements. The vidua finches are therefore extremely specialised parasites, with each species mostly exploiting a single host species. 

‘The mimicry is astounding in its intricacy and is highly species-specific,’ said Dr Gabriel Jamie, lead author on the paper and a research scientist in the University of Cambridge’s Department of Zoology. He added: ‘We were able to test for mimicry using statistical models that approximate the vision of birds. Birds process colour and pattern diff erently to humans so it is important to analyse the mimicry from their perspective rather than just relying on human assessments.’ 

A parasitic purple indigobird nestling (right) alongside two nestlings of its host, the jamesons firefinch

The imposters didn’t get everything right and there were minor imperfections in the way they copied the greenfinch chicks. The researchers believe these may exist due to insufficient time for more precise mimicry to evolve, or because current levels of mimicry are already good enough to fool the host parents. But even these imperfections could help the vidua finches. The researchers think that some imperfections might actually be enhanced versions of the hosts’ signal, forcing the parent to feed the parasite chick even more.