They might be the cutest invaders ever. It seems a population of little blue penguins in New Zealand are Australian invaders who colonised the region very recently, exploiting a gap in the ecosystem left when humans arrived and wiped out the local penguins.

Little blue penguins, known as fairy penguins in Australia, are the smallest penguins in the world, growing to about 30cm tall and weighing a bit over 1kg.

Stefanie Grosser from the University of Otago and colleagues found the replacement happened between AD1500 and AD1900, within a few hundred years of humans arriving in the 13th century. Their results were published on Wednesday in the journal Proceedings of the Royal Society B.

Ancient DNA analysis of hundreds of penguin bones from before and after humans arrived in New Zealand has shown a sharp turnover in the population, where a similar Australian species, Eudyptula novaehollandiae, replaced the local species, Eudyptula minor, in Otago on the Southern Island.

“Amazingly, all of the bones older than 400 years belong to the native New Zealand species,” Grosser said.

Looking at the genetic diversity in the population, they estimated the Aussie invaders started off with a population of about 3,000 penguins.

Jonathan Waters from the University of Otago, who led the study, says the bones they collected were from the mounds of rubbish, and were clearly eaten by the local Maori.

The Australian little blue penguins were hard to visually distinguish from the native New Zealand ones, and until recently were thought to be a single species. But Waters said they were easily distinguishable by their behaviour. “That distinct behaviour where the penguins come ashore in big rafts of 100 or 200 – that’s the Australian one,” he said. The Australian ones can breed twice a year, while the native New Zealand one can only breed once, Waters said.

Digital audio analysis also revealed they had different calls, Waters said.

“It’s a bit like the accent of saying ‘fush and chups’ rather than “feesh and cheeps’,” he said.

Whether or not humans were responsible for extinctions in New Zealand had been a matter of controversy, but the researchers said the fact the penguins lasted for so long prior to human arrival, and continued to live to the present day further south where human impacts were less, was strong evidence the local extinction was caused by humans.

Some native New Zealand little blue penguins also continued to exist further north, Waters said, where they probably recovered in more recent times.

“Many of New Zealand’s animal species, birds in particular, have suffered at the hands of people. The really exciting thing about these findings is that they show how quickly nature can respond to human impacts,” said Jonathan Waters from the University of Otago, who led the study.