Lodown, Oktas8, & disco_air
This is straight out of a one of my very old Uni physics texts.
The Problem
A container full of ducks sits on scales being weighed. If you were to scare the ducks so they took to the air and flapped about, would this reduce the overall weight?
Think about it.
The solution:
The weight, or mass of the container would remain exactly the same as long as the flapping ducks remained within the container. The lift from the ducks' wings would exactly balance their weight. This relates to Newton's third law which states that action and reaction are equal and opposite.
By the way, it is irrelevant whether we are talking about buoyancy or lift. The same principal still applies because we are dealing with mass and weight caused by gravity and Newton's Third Law. Just because one is being supported because it is lighter than its surrounding and the other is producing lift from its wings is a red herring. It makes absolutely no difference to the point I am trying to make. This question has been around since man first learnt how to fly. Since then some very very intelligent people, more intelligent than you or I, have said that if the birds become airborne in the closed environment the total weight of that environment won’t change.