Thunderbirds too slow for changing climate

However new research reveals the extinction of dromornithids may have been down to something far more mundane: that they were just too darn slow to adapt to a changing environment.

Big bones from the mighty birds excavated in the northern Flinders Ranges and near Alice Springs have yielded fresh insights into their time-heavy breeding patterns.

Microstructure studies of the fossil finds by vertebrate palaeontologists indicate the size and reproduction cycle of dromornithids gradually changed over millennia.

Yet they failed to keep pace with the world around them.

"Sadly these amazing animals ... faced rising challenges of climate change as the interior of Australia became hotter and dryer," says Professor Anusuya Chinsamy-Turan from South Africa's University of Capetown.

"Their breeding biology and size couldn't match the more rapid breeding cycle of modern day emus to keep pace with these more demanding environmental conditions."

Prof Chinsamy-Turan says determining how long the birds took to reach adult size and sexual maturity were key to understanding their evolutionary success and ultimate failure to survive alongside humans.

The earliest and largest species, Dromornis stirtoni, lived seven million years ago, stood three metres tall and weighed 600kg.

It also took up to 15 years to fully grow and become sexually mature.

The smallest and last of the flightless mihirung, Genyornis newtoni, lived in the late Pleistocene era when the climate was far drier with greater seasonal variation and unpredictable droughts.

With a body mass of 240kg, it still grew six times larger than emus but reached adulthood faster than the first thunderbirds, likely within one or two years, and started breeding soon after.

However Genyornis newtoni needed several extra years to fully grow and so its progression was still slow compared to nearly all modern birds that reach adult size in a year and can breed in the second year of life.

Flinders University co-author Professor Trevor Worthy says dromornithids were contemporary with emus long before extinction.

"They persisted together through several major environmental and climatic perturbations," he said.

"However while Genyornis was better adapted than its ancestors and survived two million years ... when arid and drought conditions were the norm, it was still slow-growing and slow-breeding compared to the emu."

Prof Worthy says their differing breeding strategies gave the emu a key advantage when their paths crossed with humans about 50,000 years ago, with the latter dying out within 10,000 years.

"In the end, the mihirungs lost the evolutionary race and an entire order of birds was lost from Australia and the world," he said.

Earlier this year, researchers revealed shell fragments found across the continent with unique burn patterns implied humans raided thunderbird nests and stole their giant eggs for food.

A fossil discovery by Flinders scientists northeast of Adelaide in 2021 also detected bone infections that may have made it increasingly difficult for Genyornis to find food and water.

Australian Associated Press