If, as the old proverb goes, a bird in the hand is worth two in the bush, it’s because most of today’s birds can swiftly fly away before you can catch them. But that hasn’t always been true: The first birds were not impressive flyers, according to a new study of fossil bird feathers. Some researchers, though, say it’s too soon to clip the wings of our earliest feathered friends.
Most scientists agree that birds evolved from small dinosaurs at least 150 million years ago, and the earliest known birds have wings and feathers that look much like those of modern birds. Yet researchers have long debated whether these first birds, such as 140-million-year-old Archaeopteryx, were capable of the kind of vigorous wing flapping necessary for sustained flight, or whether they merely glided or parachuted between treetops and the ground.
Biologist Robert Nudds of the University of Manchester in the United Kingdom and paleontologist Gareth Dyke of University College Dublin in Ireland focused on Confuciusornis, a bird dated to about 100 million years ago. They wanted to know whether its feathers were strong enough to provide both lift and rapid flapping without buckling from the effort. To figure that out, they turned to an approach called Euler-Bernoulli beam theory, which has long been used by engineers to calculate the load-bearing strength of structural beams. (Early successes of this method were the Eiffel Tower in Paris and the invention of the Ferris wheel, both in the late 19th century.)
Nudds and Dyke concentrated their analysis on the so-called primary feathers on the outer edge of the wing, which bear most of the load. Using five specimens of Confuciusornis from two museums in Germany, the researchers measured the lengths of the primary feathers and the diameters of the rachis, the long, thin shafts that make up the backbones of the feathers and which are analogous to structural beams. These measurements, along with estimates of Confuciusornis’ body weight, were plugged into Euler-Bernoulli equations to calculate how well its primary feathers would withstand the forces generated by lift and flapping. When the number crunching was done, it didn’t look good for early flight: The wings would have buckled under the stress, Nudds and Dyke report in tomorrow's issue of Science.
And when the same analysis was applied to a specimen of the older Archaeopteryx, it too failed to make the grade. Yet whereas the feathers of both prehistoric birds had only about half the strength necessary to prevent buckling, Nudds and Dyke calculated that modern birds—such as pigeons, gulls, and vultures—have feather strengths up to 13 times greater than necessary to engage in flapping flying, an impressive margin of error. The authors conclude that unless the feather structures of Confuciusornis and Archaeopteryx were greatly different from those of modern birds—for example if their rachis were solid rather than hollow like those of modern birds, a possibility they consider unlikely—then flapping flying must have been a later development.
But some researchers are not ready to close the book on early flight. “I agree that Confuciusornis and Archaeopteryx were poor fliers,” says Luis Chiappe, a paleontologist at the Natural History Museum of Los Angeles County in California. “I don’t agree, however, that these birds were unable to fly by flapping their wings.” That’s because, he says, the fossilized shafts of the feathers of early birds are often not well defined, making them difficult to measure accurately.
Philip Currie, a paleontologist at the University of Alberta in Canada, says that although the paper provides the “most convincing evidence yet” that these birds did not do well in the air, he also questions the authors’ conclusions that they were capable of only gliding or parachuting. The birds' fossils have been found both in marine and lake sediments, Currie says. “If they were only dropping out of trees, how did they end up so far from shore?”