Aliens do well around other aliens, according to new research that suggests that invasive bird species such as the common myna are more likely to thrive if there are other introduced species in the area.
And aliens — or introduced species — also do better if the climate is similar to the one they came from.
These are the findings of a large international study, published today in the journal Nature, that looked at more than 4,000 instances where birds had been introduced around the globe.
The researchers studied 700 different bird species including the common or Indian myna (Acridotheres tristis) and another species Australians love to hate: the common starling (Sturnus vulgaris).
"Nobody's ever attempted anything of this kind of scale and detail," said co-author Tim Blackburn, an ecologist from University College London.
"I never thought it was even going to be possible."
The team looked at a variety of factors in their analysis, from how many birds were introduced on each occasion, where they were being introduced, the size of their clutches, through to the relative brain sizes of different species.
"All of these different types of characteristics are important," Professor Blackburn said.
But overall, he said, environmental factors played the biggest part.
Introduced species did better if the climate was similar to the one they came from.
"If you try to introduce parrots to the Arctic you know they're going to have a bad time," Professor Blackburn said.
And introduced birds were more likely to succeed if there were already other types of aliens — not just bird species — present at the same location.
"That's obviously a worry because what it's suggesting is there's something about the presence of aliens that's facilitating more aliens arriving," Professor Blackburn said.
While the exact cause of this phenomenon is unclear, it can lead to something called an invasion meltdown where essentially the area just becomes an environment of aliens, Professor Blackburn said.
The common myna isn't our biggest problem
The study has done an amazing job of synthesising a huge amount of data and showing the complexity of the issue, said Australian Museum ornithologist Richard Major, who was not involved in the research.
However, the impacts of invasive birds in Australia are trivial, he said.
"Invasive birds in Australia are really a very minor problem in terms of biodiversity, compared with habitat loss," Dr Major said.
It's the loss and modification of habitat that is historically what has led to extinctions in Australian fauna, he said, and that trend is likely to continue.
But that hasn't stopped the common myna from getting a bad rap.
"The common myna is really the most hated bird in Australia," Dr Major said.
"People don't like them and this is really because they cohabit so much with people —they live in the places where people do, so we have a lot of contact with them around houses."
Dr Major said there's been a big expansion in the range of the common mynas in Australia in the past few decades, so while their impact on biodiversity is limited now there is the possibility they will have more of an effect in the future.
But, he said, the myna's native doppelganger, the noisy miner (Manorina melanocephala) has a much bigger impact on biodiversity loss within Australian bird communities.
This superabundant species has been able to exploit the changes humans have made to the environment. And it's also very aggressive and competitive with other native species.
Ironically, this aggression could protect Australia from having even more invasive birds, said conservation scientist Salit Kark from the University of Queensland, and a co-author on the paper.
"We think that the aggressiveness of our native species, at least in some areas, helps our native species," she said.
"However there are very specific species that are native and very aggressive, and those would be the more successful ones."
An invading species doesn't have to be from another country to cause problems.
"We have our very own species like the kookaburra which are native to Australia, but invade other parts of Australia like Tasmania and they have huge impacts," Dr Kark said.
"They are invasive species because they invaded an area that is not part of their native range."
Aliens are on the rise
"Every year we're getting hundreds of new populations of alien species being established around the world," Professor Blackburn said.
A lot of that is due to the large numbers of goods we're moving around the globe, providing ample opportunities for species to hitchhike. There are also the species we're moving deliberately because we're trading them.
The best way to prevent successful invasions is to stop alien species getting into new environments in the first place, Professor Blackburn said, which is why strict biosecurity laws like we have in Australia are so important.
"The more species you put into the environment, essentially the more opportunities you have to find a species that matches that environment," he said. "You're going to hit the jackpot."
And once an alien bird species has established a population it's very hard to eradicate them.
Australia's size and huge range of climates also increases the potential for many alien species to establish themselves here, Dr Kark said.
"Invasive birds should receive far more attention, in management and in biosecurity," she said.
"We have a huge responsibility because many of our native species, and of our threatened species, are endemic species so if we lose them they're lost."