A pair of Zebra Finches in the Australian heat. Credit:Laura Hurley The sperm was obtained through cloacal massage, where the researcher would "take the bird and gently squeeze and kind of milk out the sperm", according to associate researcher Laura Hurley from Macquarie University. From collection, the sperm would be examined under a microscope and its movement tracked by a computer program. The findings showed birds exposed to 40-degree heat daily had a severe reduction in 'normal' sperm after the 14 days, dropping from 90 per cent down to 55 per cent. According to Ms Hurley, the findings show the detrimental damage global warming is having on the avian population.

"This is a problem for birds because only normal sperm can be stored by females which has to be stored before usage. More than one sperm has to reach the egg for normal embryonic development to occur," she said. "One sperm isn't enough. Typically you have several million going in and only about 42 reaching the egg. So if you have significant damage already going in, you're reducing the chance of a proper number of normal sperm reaching the egg." The zebra finch is considered "the white mouse of bird research because they very readily breed ... and are a very ideal species to work on," Ms Hurley said. "We worked on them because they are adapted to breeding in hot conditions, and since we are interested in how climate change is affecting reproduction, they make an ideal subject," she said. This proposes a concerning issue for many other native Australian birds that aren't as adaptable and have limited breeding periods.

"If these birds that are already adapted to breed in really hot conditions are showing damage caused by heat, what is this going to mean for other species?" she said. "Think about what happened to the birds when it hit 47 [degrees] in Penrith. Those birds aren't used to those temperatures." Ms Hurley predicts that the birds in Penrith currently breeding would probably have had a reduction in normal sperm and "might not have fertile eggs laid during that time period". This is only a week after Penrith was the hottest place on earth, reaching 47.3 degrees on the Sunday. The team is looking into further research on how heat affects sperm productivity, observing hormonal and metabolic changes in the zebra finch.

It is still unknown if the heat itself is directly damaging the sperm or whether the changes in sperm morphology is due to heat stress within the bird. However, Ms Hurly strongly argued against the idea that Australia's native birds have adapted to the rising temperatures. "It is a myth. If you look at the records whenever we have these huge heatwaves, you have huge die-offs of birds. So a lot of times when you have prolonged heatwaves, that's when you start hearing about the anecdotes of the large die-offs of budgies and zebra finches because they just can't handle it," she said. "People think it's a harsh, dry environment yes, and they are built to deal with it, but if we keep making it harsher and drier then they are not going to be able to deal with it."