The number of brush turkeys has exploded in Brisbane and they have got their eyes on Sydney.

Behavioural ecologist Professor Darryl Jones said nothing - not cats or cars - was stopping the rise of the birds, which are also commonly known as bush turkeys, in urban Brisbane.

"They've increased by 700 per cent in the last 20 years [in Brisbane]," he said.

The birds had gone from being unknown in Brisbane except in a few parts, to owning virtually every suburb.

"I'm supposed to be the expert on brush turkeys and I still can't explain why that's going on," Professor Jones said.

Prior to the 1970s, brush turkeys were a major game bird and were hunted actively.

But once they were granted legal protection, they made a remarkable comeback from being an endangered species.

"Biggest news for me – I saw a brush turkey walking down the main street of [Fortitude] Valley this year," Professor Jones said.

In the bush the birds faced habitat destruction, but in Brisbane people could think they were an invading army.

John Carthew, a homeowner who lives about one kilometre from Brisbane's city, snapped 13 outside his back gate.

Sorry, this video has expired Brisbane's brush turkey explosion heads south ( John Taylor )

"I've seen 19 of them roosting in a tree just over in a park here," he said.

Professor Jones said the abundance of brush turkeys was especially surprising given that eggs were abandoned in nests.

"The chicks hatch by themselves, dig their way to the surface of the mound and spend their first 20 days of their lives absolutely alone with no parental care and no-one to tell them what a predator looks like, no-one to tell them what food looks like - nothing," he said.

"I'll be the first to admit they're not smart birds but they can actually deal with whatever been thrown at them."

It might come down to the sheer weight of numbers, with each female laying 30 eggs a year.

Professor Jones said brush turkeys were now invading Sydney "in a huge way".

"I'm constantly getting reports from down in Sydney about 'what the hell are we going to do about these big birds - they're destroying our gardens', which we've known about for 30 years," he said.

"Sydney can expect an avalanche in the next 20 years of more and more brush turkeys because they love it – somehow or other they survive."