New study explains the demise of the largest birds on the planet

Dromornithidae, known as mihirungs (the Aboriginal name for the bird) and informally as thunderbirds or demon ducks (as they are related to modern waterfowl), were a clade of large, flightless birds that towered over the Australian bush for around 30 million years.

Dromornithid bones excavated in the northern parts of the Flinders Ranges, South Australia, and near Alice Springs, Northern Territory, have provided new insights into their slow breeding patterns and how this could have contributed to their extinction.

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“We studied thin sections of the fossilized bones of these thunderbirds under the microscope so that we could identify the biological signals recorded in them. The microscopic structure of their bones gives us information about how long it took them to reach adult size, when they reached sexual maturity, and we can even tell when the females were ovulating,” says study author Professor Anusuya Chinsamy-Turan, University of Cape Town, South Africa.

“Questions such as how long it took these giant birds to reach adult size and sexual maturity are key to understanding their evolutionary success and their ultimate failure to co-exist with humans.”

The researchers compared the bones of the oldest and largest baby mihiru Dromornis stirtoni, which lived 7 million years ago, was up to 3 meters tall and had a mass of up to 600 kilograms, with the late Pleistocene species Genyornis newtoni – the last and smallest mihirung with a body mass of around 240 kilos.

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The study indicates that Dromornis stirtoni – arguably the largest bird to ever live on Earth – took a long time to grow to full body size and to reach sexual maturity, possibly up to 15 years. On the other side, Genyornis newtoni grew faster than the first mihirung, probably reaching adult size within a few years and beginning to breed soon after.

However, their growth was still quite slow compared to almost all modern birds which reach adult size in one year and can breed in the second year of life.

The last mihir cubs also shared their environment with early emus, now the world’s third largest bird.

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In the past, mihirung birds survived climate change and unpredictable droughts, but 50,000 to 10,000 years ago, humans began to burn down the bushes and hunt the large animals.

The different breeding strategies displayed by emus and dromornithids gave the emu an important advantage when the paths of these birds crossed with humans, with the last of the dromornithids becoming extinct around 40 thousand years ago.

“Unfortunately for these magnificent animals, already facing increasing challenges from climate change as interior Australia became warmer and drier, their breeding biology and size could not match the faster breeding cycle of modern (smaller) emus to keep up with these more demanding environmental conditions, ” the researchers conclude.

The article “Osteohistology of Dromornis stirtoni (Aves: Dromornithidae) and the biological implications of the bone histology of the Australian mihirung birds” is published in The anatomical record (2022). Materials provided by Flinders University.

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