NY Times article
here.

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.