"...But, said David Lentink, a Stanford University engineer, researchers had “never measured the aerodynamic force directly of any flying animal.” Dr. Lentink and two colleagues reported last month in the journal Interface that they had solved the problem.
"Scientists had found other ways to measure that force indirectly — by observing air flow, for example, and going through some complex calculations.
"Dr. Lentink built a box big enough for a bird called a parrotlet to fly from one perch to another. It was equipped with high-speed video recorders and sensors to weigh the box every millisecond.
"The sensors would register the added force of a wing beat, and by correlating video and sensor data, the researchers were able to come up with an exact measurement of the wing’s force — two times the bird’s weight on the downstroke, as it turned out.
The researchers tested their method against other methods to make sure it was correct.
"Engineering was only one part of the experiment, Dr. Lentink said. He wanted to measure natural, untethered flight, and trained the birds with treats so they would enter the box and fly to the place he pointed at.
"The birds are never touched through the whole process. “The bird actually does what it wants to do,” he said.