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.
(Read about airplanes designed to fly using thermals.)
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."
(Read related story: "Terror Birds: Predators With a Kung Fu Kick?" [August 1, 2005].)
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.
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