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 cuckoo trickery goes a lot further, according to new research, reported in the journal Science.
Female birds take on the appearance of predators to avoid being mobbed by angry parents and when a local bird population gets wise to one disguise, the cuckoo uses another to slip through their defences.
Reed warblers are known to attack female cuckoos suspected of trying to invade their nests, so to avoid the reed warbler lynch mob, many female cuckoos have 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. But cuckoos come in more than one disguise. Some females have reddish brown feathers, possibly thought to imitate other predators such as kestrels.
Professor Nick Davies, from Cambridge University, 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."
This is the first time "social learning" has been documented in the evolution of animal mimicry. Co-author Dr Rose Thorogood, also from Cambridge University, said: "Our research shows that individuals assess disguises not only from personal experience, but also by observing others.
"However, because their learning is so specific, this social learning then selects for alternative cuckoo disguises and the arms race continues."