As skies fill with millions of migrating birds, European scientists say the seasonal miracle appears to hinge on a seeming contradiction: The fatter the bird, the more efficiently it flies.
The results of their study -- involving four birds that were captured as adults and trained to fly in a wind tunnel -- contradict a central theory of aerodynamics, which predicts that the power needed to fly increases sharply with load.
For birds, apparently, the cost of flying with heavy fuel loads is considerably smaller than previously thought.
"We have measured, for the first time, how flight power changes with body mass in a bird and the results were very surprising," said Anders Kvist of Sweden's Lund University, the lead author of the study in the latest issue of the journal Nature.
Researchers found that red knot wading birds double their normal body weight of 3.5 ounces before making their twice-a-year, nonstop commute between the British Isles and the Russian Arctic. Distance: 3,100 miles.
Another Nature study -- this one involving pelicans trained to follow a motorboat and a light aircraft -- quantified the benefits of flying in an aerodynamic V formation, which allows birds to save energy by gliding in the lead bird's air stream.
Flying in formation, their heart rates were as much as 14.5 percent lower than flying solo, according to Henri Weimerskirch and colleagues from the Centre National de la Recherche Scientifique in France.
Avian researchers who did not participate in either experiment said the findings help explain how birds complete arduous migrations.
"It's always just amazed me to think if we took an airplane and doubled its weight and tried to fly it we couldn't get it to fly, and that's exactly what these birds are doing," said Brian Harrington, senior scientist at Manomet Center for Conservation Sciences near Boston.
Researchers had assumed that sleeker, more athletic birds would have the best chance of survival.
The first study suggests that building up fat deposits to be burned as fuel during the migration is more than worth the energy it takes to carry the additional weight. Heavier birds apparently use their muscles more efficiently.
Just why this is so remains a mystery, said British zoologist Jeremy Rayner of the University of Leeds.
"A central question that has occupied a lot of us for some time is how much energy it costs the bird to fly," Rayner said. "How does a bird cheat what seems like a fundamental of physics? One day we'll get the answer, but at the moment it's not obvious."