Groups Versus The Cuckoo

M G Bell
January 6, 2014

It's well known that cuckoos parasitize the nests of other birds, and the benefit to the cuckoos of this "brood parasitism" is obvious; what is not clear is why young members from other nests of the species that is being parasitized sometimes help to feed an intruder cuckoo chick.

Research carried out by a team led by evolutionary biologist Dr Naomi Langmore of the Australian National University has studied this phenomenon, known as "cooperative breeding", concluding that it is because the continued presence of a group of the host species acts to deter the cuckoos from placing their eggs in the hosts' nests in the first place.

"It's a very puzzling type of behaviour," says Langmore. While at first it was thought that the reason for the young birds to hang around was simply that they helped to raise more chicks, the research has suggested that the group mechanism is more credible.

The researchers studied colonies of speckled warblers and fairy wrens, finding that a team of four or five group helpers made it much more likely the hosts could defend their nests against cuckoos than a smaller team of only two or three birds, not being enough individuals to "mob" cuckoos and drive them off.

"Maybe the main benefit cooperative breeding provides is in safety in numbers," says Langmore.

"Brood parasites lay their eggs in the nests of other birds and then they abandon their young to the care of the host," says Langmore. "So the host loses that brood then they invest weeks or months rearing the cuckoo chick and very often have no time left to breed again after that. This is a massively costly thing for the host."

The research showed a clear correlation between the size of groups of fairy wrens and their ability to resist parasitism.

"If there are four or more individuals in the group they almost never get parasitized by a cuckoo whereas the small groups of two or three individuals are much more likely to get parasitized."

"We can't say that brood parasitism actually caused the evolution of cooperative breeding because we can't say which one came first," says Langmore. "But we can say that it provides a very strong selective force for the maintenance of cooperative breeding."



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