Largest Flying Bird Could Barely Get off Ground, Fossils Show

The largest bird that ever flew was an expert glider but was too heavy to fly by flapping its wings, researchers say.

Getting off the ground was a challenge for the 155-pound (70-kilogram) Argentavis magnificens, a condor-like bird that lived in the Andes mountains and the pampas of Argentina about six million years ago.

Despite its massive flight muscles and 21-foot (6.4-meter) wingspan, the giant bird probably could not generate enough lift to take off from a level surface, according to a new study.

Like human hang gliders, Argentavis probably had to run downhill into a headwind to become airborne, said Sankar Chatterjee of Texas Tech University in Lubbock.

"Takeoff capability is the limiting factor for the size of flying birds, and Argentavis almost reached the upper limit," Chatterjee said.

"Heavier birds such as the ostrich had to give up flight."

Once aloft, however, Arentavis was no ostrich. Despite weighing as much as 16 bald eagles, Chatterjee said, "it was an excellent glider, like a sail plane."
Riding the Winds

The new understanding of Argentavis flight comes from an unusual collaboration between paleontologists and a retired aeronautical engineer.

The researchers took measurements from Argentavis fossils and then conducted their analysis using a computer program designed to study flight performance in helicopters.

"Birds are commonly compared with aircraft, but in reality helicopters are a better analogy," Chatterjee said.
Unlike engine-powered airplanes, he noted, birds rely on their wings for both forward thrust and vertical lift, the two components necessary for flight.

Although Argentavis could not wing skyward on its own, the researchers say, it could have reached high altitudes by riding winds deflected upward over mountains.
More commonly, particularly in open terrain, Argentavis probably gained elevation by circling inside rising columns of warm air, known as thermals.

The huge flyer may have traveled hundreds of miles by repeatedly riding thermal "elevators" and then soaring gradually back to earth, Chatterjee said.

Some of the largest flying birds today, such as condors and eagles, pursue a similar strategy. Although capable of powered flight, these species save energy by letting air currents do most of the work required to gain altitude.

Lifestyle Alternatives

In the past, researchers have disagreed as to whether Argentavis was a predator, like most hawks and eagles, or a scavenger.

Chatterjee and co-author Kenneth Campbell, of the Natural History Museum of Los Angeles County, say fossil details indicate the species was an active predator.

"[The birds' skull] was adapted for catching prey and swallowing it whole," Campbell said.

"Its jaw mechanics were not suited for tearing flesh from carcasses, as in vultures, nor for tearing prey animals apart for swallowing, as in eagles and owls."

But Paul Palmqvist, of the University of Malaga in Spain, has argued that a flying species as large as Argentavis must have been a scavenger.

Palmqvist's argument is based in part on a predictable relationship between body size and foraging area seen in predatory hawks and eagles today.

Given its huge size, Palmqvist says, a predatory Argentavis would not have been able to cover enough ground and locate enough prey to meet its daily needs.

"A vulturelike behavior is more reasonable, as vultures have smaller range areas," Palmqvist said. "Carrion is more available than living flesh."

The new flight analysis, he said, also tends to support his view.

"Given its lack of maneuverability, a predator this size would have a problem landing on its prey," Palmqvist noted.

But Chatterjee and Campbell said the species was certainly a capable enough flyer to attack live prey—probably rabbit-size mammals—from the air.