A study led by ANU has discovered how a mother knows her chicks and can spot an imposter in her nest, even if it looks almost identical to her chicks.

Brood parasitic cuckoos lay their eggs in the nests of other birds and abandon their young to the care of the nest's host, leading to significant disruptions to the birds involved.

A research team from ANU and The University of Sydney studied the large-billed gerygone, a small songbird that is invaded by the little bronze-cuckoo, in Cairns, Australia. Large-billed gerygones usually evict cuckoo chicks from their nests within a few hours of hatching.

Senior researcher Professor Naomi Langmore said the study aimed to find out how the large-billed gerygone recognised cuckoo chicks. 

"Cuckoos pose a major threat to the hosts - when the cuckoo chick hatches, it evicts all the host eggs from the nest," said Professor Langmore from the ANU Research School of Biology.

She said some hosts had evolved the ability to recognise and evict the cuckoo chick.

"Theory predicts that evicting cuckoo chicks based on their physical appearance poses the real risk that the bird could evict one of its own chicks, so the bird supposedly uses indirect cues such as the invading chick being alone in the nest," Professor Langmore said.

"Our new findings indicate that the host actually has the ability to recognise and discriminate chicks based on their appearance, particularly the quantity of nestling down feathers," she said.

Professor Langmore said the findings were surprising because they showed that hosts can evolve to have true recognition of their chicks.

"We think they have an innate template of what their chicks should look like," she said.

The research is published in Royal Society Proceedings B.

Lead author Hee-Jin Noh said the cuckoo chick was a very accurate mimic of the host chicks, but had slightly fewer nestling down feathers.

"We thought the nestling down feathers might be a cue that hosts could use to discriminate between host and cuckoo chicks," said Ms Noh, a PhD scholar at the ANU Research School of Biology.

"When we trimmed the down feathers of cuckoo chicks they were more likely to be rejected.

"Host chicks are normally never rejected by their parents, but when we trimmed their down feathers some were rejected."

The strategy of brood parasites relying on others to raise their young appears among some insects and fish as well as birds.