Smart bird species unite to fight back against cuckoos’ egg scam

Cuckoos don different disguises to lay eggs in other birds’ nests unmolested, scientists have learned.

A well-known feature of the cuckoo is its ability to lay eggs that resemble those of its victims – but the bird’s trickery goes a lot further, according to new research.

Female birds take on the appearance of predators to avoid being mobbed by angry parents.

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When a local bird population gets wise to one disguise, the cuckoo uses another to slip through their defences.

Common cuckoos lay eggs in the nests of other birds. After hatching, young cuckoos throw out the host birds’ eggs and chicks to be raised by “adopted” parents.

But it does not always go the cuckoo’s way.

Reed warblers are known to attack female cuckoos suspected of trying to invade their nests.

However, to avoid the reed warbler lynch mob, many female cuckoos have now evolved grey plumage resembling the sparrow hawk, a fearsome song bird predator.

Eventually, though, reed warblers summon up enough courage to turn on the disguised cuckoo despite her scary appearance.

In this latest study, scientists found that one reed warbler attack triggers others against the same recognisable enemy.

But cuckoos come in more than one disguise. Some females have reddish brown feathers, possibly thought to imitate other predators such as kestrels.

Reed warblers on alert for grey cuckoos tend to ignore the reddish brown “rufous” variety, leaving their nests open to invasion.

Professor Nick Davies, from Cambridge University, whose findings are reported in the journal Science, said: “It’s well known that cuckoos have evolved various egg types which mimic those of their hosts in order to combat rejection.

“This research shows that cuckoos have also evolved alternate female morphs to sneak through the hosts’ defences.

“This explains why many species which use mimicry, such as the cuckoo, evolve different guises.”

To study cuckoo disguises, the scientists placed models of grey and rufous cuckoos among groups of reed warblers.

They found that reed warblers directed their attacks at the cuckoo variety their neighbours had mobbed too and as numbers of one cuckoo morph were increased, it became easier for the other to remain undetected.

This is the first time “social learning” has been documented in the evolution of animal mimicry, the researchers said.