14 January 2015 Last updated at 08:35
by Victoria Gill
Science reporter, BBC News
Scientists at Stanford University in the US have developed a super-sensitive device that can measure the weight of a bird in flight.
The invention, created by Prof David Lentink's research team, measures the force produced by every wing flap.
The device, described in the Royal Society journal Interface, will enable researchers to carry out tests of miniature drones, to assess more precisely their flight performance.
It has also answered a physics riddle.
This question, Prof Lentink explained, is whether a container or a truck carrying birds changes in weight when the birds inside were flying.
It was investigated in an episode of the US television show MythBusters; the presenters weighed a trailer while birds flew inside it, and concluded that it was no different to when the birds were still.
This new device, however, is so precise, that it shows that this is not quite right; the weight of the container would actually change as the birds flapped their wings.