The finding offers unique insights into a 20 million-year-old evolutionary arms race. Only seven groups of birds in the world have evolved as brood parasites, laying their eggs in other birds' nests, and ecologists have long been fascinated by this behaviour as an example of evolution in action.
Spottiswoode and Stevens worked on two tropical African species, the parasitic Cuckoo Finch and one of its hosts, the Tawny-flanked Prinia.
Until recently, most work on cuckoos has been done in temperate regions - Europe and North America - where species are relatively young in evolutionary terms. In the tropics, however, the Cuckoo Finch and Prinia could have been locked together in an evolutionary arms race for up to 20 million years.
As parasites have evolved ever better manipulation of their hosts, hosts have responded with ever more refined defences to evade parasitism. As a result, the Cuckoo Finch's mimicry of host eggs is extraordinary, as is the Prinias' ability to spot the parasite's eggs.
Spottiswoode and Stevens found that Prinias are amazingly good at rejecting foreign eggs, and that they use colour and several aspects of pattern to spot the parasite's eggs. Mysteriously, however, they do not seem to use the scribbles that uniquely occur only on the Prinias' eggs.
The specific traits used to distinguish foreign eggs were exactly those found to differ most between host eggs and real parasitic eggs. This suggests that natural selection is currently acting to make Cuckoo Finch eggs better mimics of their hosts', and also that Prinias use the most reliable information available in making rejection decisions.
The work was funded by the Royal Society and the DST/NRF Centre of Excellence in South Africa, and is published in PNAS (The Proceedings of the National Academy of Sciences of the United States of America) today.