ORIGINAL: kriegsmacht
..., the bird hovering inside would probably cause them to blow outwards, (or maybe suck inwards?) indicating some force acts on the sides of the container. If any force acts on the sides, wouldnt that prove that the total opposite force of the bird weight CAN't completely be upon the floor, changing the scale setting?
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The bird would indeed set up a circlulation inside a sealed container. The net result of all that air movement would be a weight-force added to the container equal to the bird's weight. Sure there are losses due to friction, heat generation, compressibility, etc, but the net result is the same.