Rising sea levels and the disappearance of an ancient, vast freshwater lake in the Murray River basin may explain why Australia's crimson rosellas developed different regional colours.
New research by the CSIRO, Deakin University and the South Australian Museum suggests the birds' three main colour forms red, yellow and orange could have evolved in response to climate change during the last ice age. The different-coloured crimson rosellas (Platycercus elegans) share a common ancestral population but are distributed across south-eastern Australia.
CSIRO's Australian National Wildlife Collection evolutionary biologist Dr Gaynor Dolman said, ''The yellow birds are found in the drier, inland areas, so the colour of their plumage may have evolved to blend in with their habitat.
''We don't know, but the redder birds might have evolved those colours so they could pick out their mates among the greener vegetation.
''Or, the colours could have something to do with what female birds find attractive.''
Published yesterday in the Royal Society journal, the research offers clues about how future climate change could affect Australia's ecosystems and wildlife.
It also overturns a previous evolutionary theory about the rosellas, known as ring theory.
Research done in the 1950s suggested the rosellas formed a population ring, encircling the semi-arid regions where the species don't occur, with the two ends of the ring joining near the headwaters of the Murray and Murrumbidgee rivers in the Snowy Mountains.
By the time this geographical spread occurs over millions of years the birds found where the ring joins are two distinct species that can't interbreed, despite a continuous gene flow, or hybridisation, between other populations around the ring.
''But we had to reject that theory because we found multiple genetic discontinuities around the ring. We found the three separate genetic groups didn't show a simple link to the geographical distribution of the three different colour forms,'' Dr Dolman said.
Blood and tissue samples from 307 adult rosellas from 92 localities were analysed. The results of DNA tests showed red rosellas on Kangaroo island were genetically similar to orange rosellas found in Adelaide, despite being separated by 15km of ocean.
But, genetic differences occurred in geographically linked yellow and orange rosella populations in inland south-eastern Australia.
And birds from Adelaide and the western slopes region near Wagga Wagga also had inexplicable similarities.
''It is possible the crimson rosellas formed a ring at some stage in their evolutionary history, but the evidence could have been lost through changes to the climate or environment,'' Dr Dolman said.
The research suggests that rising sea levels during the Pleistocene could possibly have cut off the Kangaroo Island rosellas from the mainland.
A massive 33,000sqkm stretch of water, Lake Bungunnia, may also have isolated crimson and yellow rosella populations in the Murray Basin almost two million years ago.
''It shows we still have a lot to learn about some of most our most familiar species that we take for granted. There's still a lot of evolutionary puzzles to solve,'' Dr Dolman said.