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Wild parrots in Australia are apparently picking up phrases from escapee pet cockatoos who join their flocks. Why - and how - can some birds talk?

Those strolling in Sydney's parks are being startled by squawks of "Hello darling!" and "What's happening?" from the trees.

Wild birds such as galahs, sulphur-crested cockatoos and corellas are repeating phrases passed on by domesticated counterparts that escaped or were released, says naturalist Martyn Robinson, of Sydney's Australian Museum.

The museum has received numerous reports of talkative wild birds from startled members of the public.

Birds are social creatures, and chicks learn to communicate by imitating the sounds made by their parents and those at the top of the flock's pecking order.

Unlike humans, birds do not have vocal cords. Instead, they are thought to use the muscles and membranes in their throats - specifically the syrinx - to direct airflow to make tones and sounds.

Not all birds can learn to make entirely new sounds. To date, only three groups of distantly related birds have been found to have this ability: songbirds; parrots such as cockatoos and parakeets; and hummingbirds.

"These birds are very smart birds and very social, and communication and contact is important between them," .

"So the pet bird begins to say things it's been taught by its owner and the rest of the flock learns and starts speaking too, to mimic the pet bird."

Although parrots can make noises that sound like words, they're just mimicking sounds they find appealing, says Les Runce of the UK's Parrot Society.

"It may be a nursery rhyme, a football chant, a microwave pinging or a phone ringing."

Young birds, like human babies, learn to speak or sing through imitation, says behavioural biologist Johan J Bolhuis, of Utrecht University in the Netherlands.

In research published in August in , he describes "a transitional period of early vocalisation, which is called 'babbling' in humans and 'subsong' in birds."

And, he tells the ����ý News website, parrots and some songbird species can learn throughout their lives, such as the Sydney example.

"I have studied budgerigars - small parrots - that can teach each other to speak Japanese words.

"In this and other research we found that the brains of these birds are organised in a similar way to human brains with regard to vocal learning. Also, the same genes are involved in song and speech."

He adds that birdsong has a "primitive grammar" that is quite different from the complex grammar of human language.

"Bird research can teach us a lot about the development of human speech and the problems that may occur - stuttering, for instance. So, parrots and songbirds may hold important clues as to how we humans can learn to speak and acquire languages."

Parrot fanciers keen to teach their own pretty polly to talk may have to repeat their chosen phrase over and over. But the bird may pick it up after a single listen.

"Parrots have good memories and only need to hear a sound once to reproduce it," says Runce.

"A friend's daughter had an ingrown toenail, banged it and let out an almighty shriek. Their bird has still got that one, and that was 30 years ago."