New research has found some birds choose not to reproduce so they can guard the nests of their close relatives.
"One of the mysteries of evolutionary biology is why, in about nine per cent of bird species, some individuals choose to forgo reproduction and help others raise young," says Dr Naomi Langmore of the ANU Research School of Biology.
Dr Langmore was part of a team from ANU, Cambridge University and the University of Melbourne, which explained the mystery by finding non-breeders helped drive off birds such as cuckoos which lay their eggs in the nests of other birds.
The researchers found that by choosing not to reproduce, chickless birds ensure their genes are passed on by protecting the eggs and chicks of their relatives.
"Biologists have long wondered how this strategy, termed cooperative breeding, could be evolutionarily successful," says Dr Langmore.
"Brood parasite birds, such as cuckoos, are reproductive cheats. They lay their eggs in the nests of other birds, imposing the costs of rearing on their hosts who often lose an entire brood of chicks as a result," she says.
"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."
Understanding the interaction between cooperative breeding and brood parasites also helped explain the uneven global distribution of cooperative breeding birds.
"The distribution of cooperative breeders and brood parasites is tightly linked and concentrated in two major geographic hotspots: Australasia and sub-Saharan Africa," she says.
"In these two regions, hosts of brood parasites are much more likely to be cooperative breeders than unparasitised species, suggesting that brood parasites could place pressure on host species to take up cooperative breeding to better defend themselves."