Emus may be known as one of the biggest and most intimidating Australian bird species, but once upon a time they could be found at just two-thirds of the size.

Key points: Researchers have found small emus lived on Australia's offshore islands in the 19th century

Researchers have found small emus lived on Australia's offshore islands in the 19th century They were about two-thirds of the size of Australian mainland emus

They were about two-thirds of the size of Australian mainland emus They were found on islands including Kangaroo Island, King Island and Tasmania

A study from the University of Adelaide has found emus that lived isolated on the country's offshore islands until the 19th century were smaller versions of their larger mainland relatives, and their size correlated to the size of the islands where they lived.

The findings, published in the Biology Letters journal analysed the DNA and bone measurements of the now-extinct smaller emus and found that the smaller emus lived on offshore islands, including Kangaroo Island, King Island and Tasmania.

The large Australian emu is the only living representative but fossil and bone evidence suggests there were three other island forms of the bird which became extinct in the 19th century.

The Australian mainland emu is one of the largest native birds in the country. ( ABC Open contributor Ruth Sandow )

Lead author of the journal, Dr Vicki Thomson from the University of Adelaide's School of Biological Sciences said smaller emus on King Island had been shown to be the same species of the Australian mainland emu.

But not much was known about the evolution of the other versions.

"Our results have shown that all the island emus are genetically closely related to the much-larger mainland emu," she said.

"The smallest, the King Island emus, are typically two-thirds of the size of our mainland emus, with others ranging upwards according to the size of their island.

"The leg bones, a measure of overall body size, show size differences from the smallest, found on King Island, to the largest, on the mainland, with Kangaroo Island and Tasmanian emus in between."

Dr Kieren Mitchell, co-author and from the University's Australian Centre for DNA said the smaller size suggested that island size and reduced food may have been a factor in the evolution of the species.

"Our results have shown that all the island emus are genetically closely related to the much larger mainland emu," Dr Vicki Thomson said. ( Gary Rivett: ABC News )

"This suggests that island size, and presumably the associated reduced food resources available, may have been important in causing smaller body size in island emus," Dr Mitchell said.

But Dr Thomson suggested more work was needed to confirm the role of island size in the smaller stature of the extinct island emus.

"We do know that prior to European arrival, Kangaroo Island, King Island and Tasmania had these smaller-bodied emus and they would have been isolated from the mainland after sea levels rose around 10,000–15,000 years ago," she said.