A research partnership between ANU, the University of Melbourne and Cambridge University has shed new light on the peculiar biological phenomenon of co-operative breeding.

''One of the mysteries of evolutionary biology is why, in about 9 per cent of bird species, some individuals choose to forgo reproduction and help raise others' young,'' said Dr Naomi Langmore, of the ANU Research School of Biology.

Dr Langmore became interested in studying this breeding phenomenon after observing the aggressive behaviour of the fairy-wren towards the Horsfield bronze cuckoo in Canberra's Campbell Park.

Cuckoos are the only type of bird in Australia that lay their eggs in the nests of other birds, imposing the cost of rearing a foreign chick on the hosts, often resulting in the loss of their own brood of chicks.

''Brood parasite birds, such as cuckoos, are reproductive cheats,'' said Dr Langmore. ''The cuckoos are actually lethal. As soon as the cuckoo chick hatches, when it's naked and blind, it will balance the fairy-wren's eggs on its back and push them out of the nest.''

Brood parasites like the cuckoo can destroy a nest of offspring, but Dr Langmore said further damage is done when host birds are excluded from participating in the breeding cycle while they take on the responsibility of rearing an invading chick.

''A cuckoo is an even more of a threat than a predator. The fairy-wrens lose their chicks but also lose their breeding season.''

Dr Langmore found in many cases a breeding couple of fairy-wrens were supported by two or more non-breeding male wrens. The flock of birds works together to mob threatening cuckoos and share the responsibility of protection and feeding chicks in the nest.

''Hosts were able to escape parasitism more frequently when they mobbed together in a large group, rather than defending their nest as a breeding pair,'' said Dr Langmore. ''There is some evidence that fairy-wrens learn as they get older and experienced hens are more likely to abandon a cuckoo chick.''

The study's findings are hoped to contribute to a better understanding of this biological phenomenon and start to answer why co-operative breeding occurs most commonly in Australasia and sub-Saharan African regions.