Birds Got Too Fat to Fly After Dinosaurs Vanished?

It's been long thought that the ancestors of ostriches, emus, and other flightless birds that once flew were flightless too. But a new study says that they only began exploring the ground in earnest after dinosaurs were wiped out about 65 million years ago.

The sudden disappearance of dinosaurs opened up new, predator-free niches, where food was plentiful and flight wasn't needed for quick escapes, said study leader Matthew Phillips of the Australian National University in Canberra.

The birds then got so plump that they became too heavy to fly, whether they wanted to or not, the study suggests.

Using fossil DNA, Phillips and colleagues analyzed the genome of a giant moa, an extinct flightless bird that lived in what is now New Zealand.

(Related: "Extinct Giant Bird Doomed by Slow Growth, Study Says.")

The team found that the moa's closest relatives were tinamous—small ground-dwelling birds still found in South America that can barely fly.

During most of the Cretaceous period (146 to 65 million years ago), South America, New Zealand, Australia, and Antarctica were joined as part of the massive southern continent Gondwana. About 80 million years ago, New Zealand drifted away from Gondwana.

The researchers suggest that a moa ancestor may have flown from another location—possibly what would become South America—to New Zealand, where the bird hopped to the ground and eventually evolved into the moa.

Evolutionary Turning Point

That flightless birds evolved species by species, as the new research indicates, challenges a previous theory that flightless birds evolved from a common flightless ancestor, the study says.

"We've known what a big impact [the extinction] had on dinosaurs, but didn't know about birds and mammals," Phillips said. "This research provides a signal that it was an important point for the evolution of modern birds."

The new findings also appear to solve the ancient mystery of how flightless birds ended up on various continents.

"There used to be odd ideas about how [supposedly flightless] birds got over marine barriers," Phillips said. "But the fact that these birds all had independent ancestors that could fly explains how they got to different land masses—they flew."

The research appears in the January edition of the journal Systematic Biology.