ORIGINAL: kriegsmacht
...6 ounce hummingbird rests on perch inside 1 pound container on scale. Scale reads 1 pound, 6 ounces. Hummingbird takes flight and hovers. Scale still reads 1 pound 6 ounces when you average any fluctuations. I find this one a little hard to swallow, but it seems to be the popular opinion here.
I understand all these to be true then. Am I correct?
Hummingbird or Swallow?
Yes. The hummingbird is acting on the air which is in turn acting on the container. It's no different in effect (barring the bird's momentary acceleration) whether the bird is perched on a stick mounted to the container or it is being held up by its action against the air constrained by the container.
My reasoning is this: If we designed a scale that could capture and measure the force of ALL the air that the bird moved in order to hover, the force would equal the weight-force of the bird, same as if it was perched on the scale. Add to this the weight of a container, put the bird and enough air insided it, and you have the above case.