Bula
Wrong Wrong Wrong. The downward force of the air has no effect on the weight. Let me use the water model again to illustrate. This time we will use a deep pool with a diver wearing fins. The diver is weighted so as to give him negative buoyancy. The weight of the pool consists of the pool, the water and the diver and all his gear. Now for the diver to remain on the surface he has to expel energy by kicking with his fins. This causes a downward flow of the water beneath him, which hits the bottom of the pool. Does the weight of the pool and its contents change because of this? No, of course it doesn’t. To be clear what is happening here, the downward force is cancelled by the upward force of the diver in a sealed environment, i.e. equal and opposite forces. Basic rule of physics. This is exactly the same as the birds in flight. As for the birds being inverted? No difference. Equal and opposite forces in a closed environment.