The wings were willing, but the feathers were weak. Delicate, thin-shafted plumage would have made flapping difficult if not impossible for two prehistoric birds, a new analysis of fossil feathers suggests.

Their feathers probably would have buckled or snapped during strong flapping or sharp maneuvers, so the primitive birds may have been limited to gliding, says Robert Nudds, an evolutionary biologist at the University of Manchester in England. He and paleontologist Gareth Dyke of University College Dublin report an engineering analysis of feathers from the ancient birds Archaeopteryx and Confuciusornis in the May 14 Science.

Nudds and Dyke used a simple formula often applied to bridges and beams to estimate the load-carrying capacities of the birds' feathers, based on fossil remains. The team also looked at the feathers of four modern birds with a variety of feather and flight types – a pigeon, a gull, an albatross and a vulture.

Even though the feathers of Archaeopteryx and Confuciusornis were about the same size as those of a modern-day pigeon, they had smaller diameter shafts that rendered them much weaker.

"Even if these feathers had solid shafts, they're not very impressive," says Lawrence Witmer, a paleontologist at Ohio University in Athens who was not part of the new study. "They're so flimsy that they couldn't have supported much weight."

In straight and level flight, the lift generated by a bird's wings, tail and other flight surfaces must support the bird's weight. But during extreme maneuvers such as high-speed turns – analogous to a fighter pilot "pulling g's" – the forces on a bird's feathers are much higher, Nudds says. In those cases, birds rely on their bones and feathers having a "margin of safety" that makes them several times stronger than needed for straight and level flight.

In modern birds, feathers typically fail when forces acting perpendicular to the central shaft cause that load-bearing structure to buckle, Nudds says. To prevent this, lift-generating feathers in present-day birds are many times stronger than necessary for level flight, from a factor of around six in vultures to a factor of more than 13 in gulls. But in the ancient birds, margins were much smaller: 2.9 for Confuciusornis and four for Archaeopteryx. If these birds had feathers with partially hollow shafts similar to those of modern feathers, these margins could have been even lower, the team argues.

The ancient birds may have simply glided from one branch to another, the researchers say, or "parachuted" from high spots to low by splaying their wings and slowing their descent.

Other recent studies of Archaeopteryx – a fossil iconic of the transition from dinosaurs to birds – have also cast doubt on the creature's flying ability.

Research suggests, for example, that although Archaeopteryx had large enough feathers for flight, it didn't have the right bone structure to take the large upstroke required for efficiently powered flight, says Richard Prum, an ornithologist at Yale University.

"Not only is the shoulder joint oriented wrong for powered flight in Archaeopteryx and Confuciusornis, but the new study shows that even the feathers aren't built right for it," says Phil Senter, a paleontologist at Fayetteville State University in North Carolina. "I've thought for some time that the feathers of nonavian dinosaurs and [primitive] birds were primarily display structures, and the lack of powered flying ability is consistent with that idea," he notes.

As far as Archaeopteryx is concerned, Witmer concurs, to a degree. In recent studies, Archaeopteryx "has become less birdlike … and is starting to look like just another feathered predatory dinosaur." He notes, though, that at this point the features of Archaeopteryx still seem closer to birds than to other dinosaurs.

The lack of modern flying ability shouldn’t cast poor light on Archaeopteryx, Witmer adds. "A lot of ancient birds were probably pretty clumsy." The ability to glide or parachute from one branch to another was still an advantage, he suggests: "Anything that slows an organism's descent would add to its survivability."

It's possible, Witmer notes, that some aspects of the primitive feathers yet to be recognized by scientists compensated for their structural weakness.

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Image: Composite of Todd Marshall*