Evidence showing many birds were skilled fliers millions of years earlier than scientists thought has recently been unearthed in China.

The fossil of a small bird and a similar fossil found in Spain a few years ago help fill a major gap in avian evolution. They suggest that birds that could fly were both numerous and diverse in the Cretaceous Period, 70 million to 140 million years ago.The latest fossil, estimated to be 135 million years old, was found in a farmer's field in Liaoning Province in northeastern China. It indicates an agile, sparrow-sized bird that could perch in trees and dart through the air to snare insects.

"If you saw this bird perched in a tree it would probably appear modern, with flying and perching abilities virtually identical to today's birds," says paleontologist Paul Sereno of the University of Chicago. Sereno, who led the painstaking reconstruction of the fossil at the University of Chicago and the Field Museum, notes three features that are also seen in modern birds:

A keeled breastbone, a short tail that shifts the bird's balance over its wings and grasping feet.

The Chinese fossil is about 10 million years older than the one found in Spain and just 10 million to 15 million years younger than archaeopteryx, the oldest known birdlike animal.

"Discovery of the Spanish bird and now our fossil indicate flying and perching birds must have been prevalent on land areas in the early Cretaceous," says Sereno, whose work has been supported by the National Geographic Society.

"The Chinese bird was found in sediment that once formed the bottom of a freshwater lake, which was at least 200 miles from an ocean. Most of the other avian fossils found later in this period belonged to ocean shore birds that could neither fly well nor perch in trees."

Blank evolutionary pages have separated 145-million-year-old archaeopteryx and 70-million-year-old hesperornis and ichthyornis, the earliest other known birds. Hesperornis was a flightless animal found in chalk beds in western Kansas, once an inland sea. The toothed, loonlike swimmer may have given birth to live young and may never have come to dry land. Its wings, no more than stubs, were useless for flight.

Ichthyornis (fish bird), the first previously known bird with wings developed for sustained flight, was the size of a gull, had teeth and probably ate fish. It thrived in areas of Kansas and Texas.

A cross between a bird and a reptile, crow-sized archaeopteryx combined the attributes of both. It had the clawed fingers of a dinosaur, the scaly head and long bony tail of a lizard, and the toothed jaw of a crocodile.

Archaeopteryx (ancient wing) lived when the biggest of the dinosaurs roamed the earth about 140 million years ago. Although it may have inhabited other parts of the world, its bones have been found in only one region of southern Germany.

Since all modern birds that fly have asymmetrical flight feathers like those of archaeopteryx, it is logical to believe it too could fly. But experts still debate whether archaeopteryx was a feeble flier or could even take off from the ground. It had the large wishbone that aids powered flight, but possibly lacked the hollow bones and keel-shaped breastbone that made its descendants master fliers.

A new theory holds that archaeopteryx was a good flier that muscled its way into the air. "Archaeopteryx physiologically was still a reptile, but it had the muscle power of today's birds," says John Ruben, an Oregon State University zoologist.

He theorizes that muscle power compensated for skeletal shortcomings and made it a skilled short-distance flier. "It probably could flap around for two or three minutes at a time, but it certainly didn't undertake any long distance migrations," says Ruben.

The Chinese bird may be distantly related to archaeopteryx. "Based on what I've seen in detailed photographs, elements of the pelvis retain some of the same characteristics as archaeopteryx," says John H. Ostrom, a Yale University geology professor. "At the same time, it has developed modern birdlike features characteristic of a good flier."

Describing the fossil as "very exciting," Ostrom says its import is that "either the changes making a modern bird occurred very rapidly or modern bird lineages predate archaeopteryx. There'll be a lot of discussion about this."

Experts agree that as the epochs succeeded one another, climates and landforms changed and new birds evolved. Many familiar species appeared about 60 million years ago: herons, ducks, vultures, hawks, grouse, cranes, owls.

Songbirds, the most recent group to make their debut, emerged about 30 million years ago.