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In their fightback against the forgers, grass-warblers have become skilled quality controllers, rejecting eggs that differ from their own in colour and pattern, and all four species have evolved the ability to deposit unique вЂsignaturesвЂ™ onto their own eggs to enhance their detection of intruders. The team collected DNA samples from 196 cuckoo finches from 141 nests belonging to the four grass-warbler species and studied the majority by sequencing thousands of short segments across their genomes. A cuckoo finch chick (top) begs for food very vigorously as soon as it hatches, outcompeting the host parentsвЂ™ own chicks - here zitting cisticolas - which typically soon die of starvation. вЂњIn particular, parasites face a daunting challenge because some host species have in return evolved an astonishing diversity of egg colour and pattern вЂsignaturesвЂ™, that help hosts to distinguish their own eggs from parasitic mimics.вЂќД¬uckoo finch and host chicks. вЂњWhile maternal inheritance has allowed cuckoo finches to exploit multiple host species, itвЂ™s likely to slow their ability to evolve counter-adaptations as their hosts evolve new defences.

Dr Spottiswoode said: вЂњIn this particular coevolutionary arms race between species, natural selection has created a double-edged sword.' Such mimicry dupes host parents into accepting a parasitic egg as their own rather than throwing it out of the nest, and so has been crucial to the success of these African birds.Д«ut the researchers believe that this long-established вЂgenetic architectureвЂ™ of maternal inheritance may come back to haunt the cuckoo finches. Such вЂ maternal inheritanceвЂ™ allows cuckoo finches to side-step the risk of inheriting the wrong mimicry genes from a father raised by a different host, and so has allowed distinct lineages of cuckoo finch females to evolve specialised egg mimicry of several different host species. The research reveals that female cuckoo finches inherit their ability to mimic the appearance of their hostsвЂ™ eggs from their mothers, via the female-specific W chromosome (analogous to the male-specific Y chromosome in humans). The study, published today in PNAS, focusses on the genetics of egg mimicry in the cuckoo finch, a species which adopts a brood-parasitic lifestyle and exploits many species of warbler across Africa. Now genetic research by an international team led by Professor Claire Spottiswoode from the CambridgeвЂ™s Department of Zoology and the FitzPatrick Institute of African Ornithology, University of Cape Town and Professor Michael Sorenson at Boston University, has made a major breakthrough, and their findings may be bad news for the egg forgers. These questions have been puzzling scientists for more than a century. How then can a single brood-parasitic bird species simultaneously mimic the eggs of several different bird species to trick them into raising their young? And how do these parasitic forgers pass this ability on to their young despite interbreeding between birds raised by different hosts? Many brood parasites achieve this by mimicking the colours and patterns of their hostвЂ™s eggs, but some exploit the care of several different host species whose eggs all look different.

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This lifestyle, termed вЂњbrood parasitismвЂќ, has many advantages but also presents challenges such as how to convince the other species to accept a foreign egg.

Around the world, many birds side-step the costs of parenthood by laying their eggs in the nest of other species.

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