DeltaFree: I love the analogy with the birds in a container, but I don't think it's right
Correct me if I'm wrong, but when you use the word 'weight' then you are considering the definition of weight which includes its correlation with resistance. I.e. weight only acts on a surface, it is gravity based. If there is no reaction (surface) then there is no 'weight'.
For instance, two men standing on a weighing scales, one on top of the other! When the top man levitates away from the guy underneath, the scales will reflect a lesser weight!!
The same happens with your birdies, in that when they take off and fly inside the container which is on a weighing scale, the scales will indicate a reduced figure
When a helo takes off away from the ground, it weighs nothing
What doesn't change is the MASS. This remains the same.
So the mass of the container and birds in your analogy is 2 tonnes, and will always be 2 tonnes. The weight of the combined lot is 1 tonne (birds in flight).
That was one reason why all up weight (AUW) for aircraft was changed to all up mass (AUM) all those years ago, so that it represented more closely what was going on....
What do you think?