By Victoria Gill
Science and nature reporter, BBC News
Cuckoos and sparrow hawks have almost identical patterns on their underbellies
Cuckoos have evolved plumage patterns that give them a hawk-like appearance to scare the birds whose nests they invade, say scientists.
A study has shown that reed warblers - a cuckoo host species - are less likely to attack more "hawk-like" cuckoos.
This helps the parasitic birds to lay their eggs undisturbed.
The visible similarity between cuckoos and sparrow hawks was already clear, but this is the first study to show the effect the trickery has on host birds.
The findings are published in the journal Behavioural Ecology.
Cuckoos are brood parasites; after laying an egg in another bird's nest, they leave the unsuspecting host to raise their chick.
To fool their victims into caring for their young, cuckoos first need to lay an egg in the right nest, without being "mobbed" or attacked by the nest's owner.
"We noticed in another experiment that great tits and blue tits were just as afraid of cuckoos as they were of sparrow hawks," explained Dr Justin Welbergen from the University of Cambridge, UK, who led this research.
Since these species are not targeted by cuckoos, they have no reason to fear them, so Dr Welbergen decided to find out if a cuckoo's appearance might have evolved to help it to scare its hosts.
To study this, he and his colleagues placed stuffed dummies of cuckoos and sparrow hawks near reed warbler nests.
"The most striking similarity between a cuckoo and a sparrow hawk is the bar patterns on [both species'] bellies," Dr Welbergen explained.
So he and his team covered the bellies of the cuckoo and sparrow hawk models with white cloth, to hide their barred bellies. They then studied the reed warblers' reactions to both the "unbarred and barred" models.
"We found that the reed warblers were more reluctant to approach the birds if the dummies had barring on their chests," said Dr Welbergen.
"So if we made a cuckoo look less hawk-like - if we removed this bar pattern - it was more likely to be mobbed."
Dr Welbergen said this showed how "hawk mimicry" helps cuckoos to gain better access to reed warblers' nests in order to lay their eggs.
The findings represent just one part of an evolutionary arms race between cuckoos and the species whose nests they target.
The birds lay parasitic eggs that are very similar in colour to the host's own eggs, and a recent study found that cuckoo chicks have evolved to mimic the calls of the hosts' offspring.