Ostriches can't fly - so how did their ancestors cross oceans?

For decades, scientists scratched their heads over an unusual mystery. How did ostriches, emus, cassowaries, kiwis, and rheas manage to appear on separate continents despite being flightless? The birds belong to the paleognath family, yet most of them are unable to fly.

Their spread across Africa, Australia, New Zealand, and South America has puzzled researchers for generations. The one exception today is the tinamou, a shy bird from Central and South America.

It can manage short bursts of flight, mainly to escape predators. Still, its limited ability left scientists questioning how this bird family ended up distributed across the world. The answer, it turns out, lies in their ancient ancestors.

When ostriches could fly

Researchers once speculated that the birds’ ancestors were divided when Gondwana broke apart about 160 million years ago. This breakup formed the southern continents we know today. But genetic research pointed to a different timeline.

“The evolutionary splits between paleognath species happened long after the continents had already separated,” explained Klara Widrig of the Smithsonian National Museum of Natural History.

Her team decided to look deeper, turning to fossils of a group known as lithornithids, the oldest paleognaths discovered so far. Unlike modern relatives, these birds appear to have retained true flight ability.

Ostrich fossil shows ability to fly

Lithornithids lived during the Paleogene period between 66 and 23 million years ago. A crucial fossil of Lithornis promiscuus, first unearthed in Wyoming, provided new clues. Unlike many fragile bird fossils, this one remained intact.

“Because bird bones tend to be delicate, they are often crushed during the process of fossilization, but this one was not,” Widrig said. “Crucially for this study, it retained its original shape.”

That preservation allowed researchers to scan its breastbone, where the flight muscles would have attached. Their analysis confirmed that Lithornis promiscuus could fly, either through continuous flapping or by gliding over long stretches.

Strong power to fly

The team’s quantitative analysis revealed something more. Unlike tinamous, which rely on rapid, anaerobic bursts, Lithornis had the skeletal features of an aerobic flier. Its sternum shape aligned with birds that are capable of sustained flapping or flap-gliding, styles linked to long-distance dispersal.

The study concluded that Lithornis was not specialized for short, frantic takeoffs but instead could have crossed significant barriers. This places it among birds capable of colonizing distant lands, explaining how paleognaths spread before losing flight independently on different continents.

Abandoning the skies

If their ancestors could fly, why did their descendants abandon the skies? Widrig offered a clear explanation.

“Birds tend to evolve flightlessness when two important conditions are met: they have to be able to obtain all their food on the ground, and there cannot be any predators to threaten them,” said Widrig.

Fossil studies also revealed that lithornithids likely had a bony organ on their beaks, making them adept at capturing insects. This adaptation would have suited a ground-feeding lifestyle.

Life after dinosaurs

The second condition, lack of predators, may have emerged after dinosaurs went extinct 65 million years ago.

“With all the major predators gone, ground-feeding birds would have been free to become flightless, which would have saved them a lot of energy,” Widrig said.

At first, only small mammals remained, posing little threat. Over time, mammals evolved into predators, but by then the paleognaths had already adapted.

Some became fast runners like ostriches and rheas. Others evolved into imposing species, such as cassowaries, known for their strength and dangerous kicks.

Ostriches spread without flying

The findings support a broader evolutionary theory known as the cycle of vagility. This idea suggests that highly mobile ancestors disperse widely, only for their descendants to later evolve into more sedentary or flightless forms.

In the case of paleognaths, early flying relatives like Lithornis likely colonized distant lands. Their descendants then adapted locally, sometimes losing flight but thriving through speed or size.

Evidence from other bird groups supports this view. For example, some modern herons and egrets show striking dispersal patterns, moving across oceans and continents. Fossil and skeletal similarities suggest lithornithids may have been just as capable.

This study, published in the Royal Society’s Biology Letters, brings new clarity to an ancient puzzle. The story of the ostrich’s journey across oceans is not one of continent-splitting accidents, but of ancestors who once took flight.

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