Cuckoo finches (Anomalospiza imberbis)
reproduce by stealing something very unusual from other birds: parenting skills. The cuckoos lay their eggs in the nests of prinias and cisticolas, hoping their eggs will pass scrutiny and be incubated. Even though the host birds are able to detect minute differences between eggs and will eject foreign eggs from their nests, cuckoo finches get by following the same brood route as around 100 species of birds worldwide that have the same reproduction strategy.
The host birds have striking variations in the patterns and colours of their eggs from one individual bird to another as a defence mechanism against brood parasites. The base colour ranges from white and cream to brown through pink to olive and even green and blue-green while the eggs are further adorned with reddish-brown squiggles, blotches and spots.
Cuckoo finches have developed a strategy that confuses the host bird. In addition to specialising in mimicking the egg patterns of just one kind of host bird, by placing multiple eggs in the host nest the selected foster parent is confounded and forced to accept the con rather than reject its own eggs by mistake. This is a fatal blunder as the brood parasite’s offspring will dominate in the nest, causing the host’s brood to starve to death.
To mimic the egg patterns of a single host bird is an unbelievable achievement in its own right, as it’s not quite clear how a female cuckoo finch manages to keep laying one egg design, even after breeding with different males.
The Cuckoo finch is a member of the same family (Viduidae) as another group of African brood parasites, the indigobirds and whydahs, but it is the only species in its genus which split off from the indigobirds and whydahs about 20 million years ago.
Unlike the indigobirds and whydahs, which have since split up into numerous species each specialising on a single host species, the Cuckoo Finch has remained one species comprising several host-specific races.