For more than a century, scientists have debated how birds evolved flight. Some thought birds had ground-dwelling ancestors, developing flight by taking off from the ground. Others figured birds evolved from tree-dwellers, developing flight by first gliding from branches.
It now seems early birds might have preferred life on the ground.
In the past 15 years, researchers have uncovered a wealth of fossils of dinosaurs that could help settle the controversy and are already revealing more about how early birds lived roughly 150 million years ago. These creatures were seemingly on the way to becoming birds—"intermediates that had feathers, that were developing wings, that were forming beaks," said functional anatomist Christopher Glen at the University of Queensland in Australia.
Initially, Glen and colleague Michael Bennett looked at toe claws from 249 species of modern birds. They found that the more hooked the claws were, as is the case with woodpeckers, the more the birds preferred foraging in trees, "as curved claws would provide them a better grip there," Glen explained.
The scientists then looked at how curved the claws of early birds and their immediate bird-like dinosaur ancestors were. These all resembled the straighter claws of modern birds that favored the ground, such as those of turkeys, roadrunners and ostriches.
"I was actually expecting to find something different—I thought the 'trees-down' scenario made a lot of sense, with flight evolving as you had species jumping from tree to tree, gliding in between," Glen told LiveScience.
Glen noted many modern birds do not live solely on the ground or in trees, but rather spent time doing both, suggesting the same might have been true of early birds.
"We're not saying we can resolve the entire story about bird flight just by looking at one feature of the anatomy," he said. "But their claws do suggest they spent more time on the ground."
Still, evolutionary biologist Kevin Padian noted the controversy over how birds evolved flight. "Chickens can climb trees, but they have flat-footed claws," he said. "All these results wisely say is that the first birds had no obvious specializations of their claws [that are] characteristic of birds living in trees today."
Glen and Bennett will detail their findings in the Nov. 6 issue of the journal Current Biology.