Australasian Science: Australia's authority on science since 1938

Cuckoos Trick Other Cuckoos

Australian cuckoos are engaged in an arms race, competing with each other to take over the nests of other species.

“Previous research has focused on how cuckoos try to trick their hosts,” said Dr Ros Gloag of the Australian National University. “What our study shows is that cuckoos have evolved traits to try to trick other cuckoos, too.”

Cuckoos the world over fool other birds into rearing their young by laying an egg in the other bird’s nest. They remove just one of the existing eggs so as to not arouse the host mother’s suspicions.

The eggs of many species of cuckoo have also evolved to mimic those of the host species, which further serves to fool the host mother. The young cuckoos hatch early and dominate the brood by pushing other chicks out of the nest.

However, there is so much competition between Australian native bronze-cuckoos that they have changed the way they disguise their eggs. Unlike cuckoos on other continents, the native cuckoo’s eggs do not mimic the host’s eggs. Instead, they have evolved new ways to hide their eggs from subsequent cuckoo invasions.

“These cuckoos target species that build domed grass nests that are quite dark inside,” Dr Gloag said. “The cuckoo’s eggs are coated in a dark pigment, which makes them hard to see in the nest.”

The host mother seems not to notice that one of her eggs has apparently changed colour. However, if a second cuckoo arrives to also oust an egg and lay one of hers in its place, it may not see the first cuckoo’s dark egg, and instead throws out the host’s more visible lighter-coloured eggs.

Gloag used models of eggs of different shades to study the behaviour of the birds. “When a cuckoo sneaked in to lay an egg, it evicted a dark-coloured egg model much less often than either a light-coloured model egg or a natural host egg,” she said. “This shows that dark colouring protects a first-laid cuckoo egg, because it will hatch first and oust all the other eggs in the nest, including the second cuckoo’s egg.”

The findings have been published in Proceedings of the Royal Society B.