The researcher outfitted 26 birds with tiny accelerometers, which collected data on their flapping speeds from takeoff to landing.
Comparing the data across species, Sato found that the flapping speeds required for a bird to take off and then stay cruising are linked to its body size.
He and his team calculate that 22 pounds (10 kilograms)—the weight of a large wandering albatross—is the "pragmatic limit" for safe and sustainable flight in variable wind conditions.
Sato's findings, appearing tomorrow in the journal PLoS ONE, also suggest that, in theory, a soaring flyer can weigh no more than 90 pounds (41 kilograms) with a wingspan no wider than 16.7 feet (5.1 meters).
Other scientists are not quite convinced that Sato's research means large pterosaurs couldn't get airborne.
"One possibility is that Sato's findings don't really apply to pterosaurs or even to all birds," suggested Davin Unwin, a paleobiologist at the University of Leicester in the U.K.
For example, Argentavis, a giant bird thought to have existed six million years ago, had a wingspan of 20 feet (6 meters) and seems to have been able to fly, Unwin said.
What's more, Unwin said, giant pterosaur fossils all seem to have extraordinarily thin bone walls, which could mean the animals were lighter than their size would suggest.
Makoto Manabe, a senior scientist at the National Museum of Nature and Science in Tokyo, also thinks it's possible that pterosaurs were simply lighter than we currently think.
Or, if pterosaurs couldn't fly, Manabe wonders whether they might have been swimmers, "using their wings as fins like penguins."
"Having said that," he added, "their wings do not look very efficient at swimming."
According to study leader Sato, it's possible heavy pterosaurs overcame their difficulties during takeoff by launching themselves from high places such as trees or cliffs.
But if pterosaurs really were capable of sustained flight, "we must think about the possibility of drastic change in other environmental factors, such as much lighter gravity or much denser air over geological time," he said.
In general, Sato thinks the reactions from paleontologists have been "not so negative," despite the fact that his conclusions would bring huge pterosaurs abruptly down to Earth.
He is expecting his biggest critic to be much closer to home.
"My six-year-old son, Takuto, is a dinosaur freak," he said, "and will never agree with my findings."
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