About that bird in the aircraft...
Milt says: "Interesting to explain wing lift scientifically/mathematically/physically or whatever but the bird flying around in the pressurised cabin of an aircraft nevertheless transfers its weight to the aircraft by air pressure waves and the aircraft then transfers its weight, which includes that of the bird, to the earth's surface by more pressure waves."
OK, so when the bird is flying its weight is transferred by "air pressure waves" - maybe - and when the bird sits in seat 18B it is effectively part of the aircraft. So far, maybe, so good.
So how much does the combination weigh while the bird is descending inside the aircraft with its wings folded? There are then no "air pressure waves", and it is not "part of the aircraft".
Or try this. Suppose the bird is in its seat on the aircraft sitting stationary on the ground. We weigh the combination and get a figure. Then the bird takes to the air inside the aircraft. How much do the scales now register? Does it matter if the cabin has a roof or not? And if we (highly hypothetically) performed the same experiment with the aircraft in flight, now does it matter if there is a roof or not?