If you're driving a truck full of birds, can you bang the truck and lighten your load as the birds launch into the air? Mythbusters concluded "no" when they tackled this question several years ago—but that's because they didn't have equipment as high-tech as these Stanford engineers'. The real answer, it appears, is a bit more complicated.

The answer is "yes," but only briefly and then the birds end up weighing twice as much for a brief moment, too, as the downstroke of their wings generate vertical force.

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Ordinary scales don't work instantaneously, so when weighing the avian load they miss the fast but substantial fluctuations generated by a bird flapping its wings. To capture changes over a fraction of a second, a group of Stanford engineers rigged up a box where each wall could measure fluctuations in air pressure. It look measurements 100 times a second. Then, the researchers let a bird into the box, as New Scientist explains:

By filming Pacific parrotlets (Forpus coelestis) taking off and landing inside the box, and measuring the forces the birds generated as they did so, Lentink's team confirmed that generalist birds like this generate almost no vertical force when they flap their wings upwards – meaning they really are effectively weightless.

On the downstroke, though, the birds push on the surrounding air so forcefully that they generate a vertical force of up to twice their body weight.

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So there you have it: The answer to how much flying birds a truck weighs is somewhere between zero and twice their usual weight. It's not a magical weight-changing truck, just some weird physics. [New Scientist]


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