Lodown
Please don’t get confused by the pressure exerted on the airframe by a pressurised aircraft. We are not talking about that. We are talking about the weight of the air because of its mass x Gravity. Let’s look at a non pressurized aircraft if it helps to show what I am talking about. The total weight of that aircraft in flight is the BEW + crew + fuel + payload (pax and freight) + air in the cabin. Because the air is always in the cabin its weight is actually taken into account in the BEW so as pilots we don’t see or think about it. If some of the freight are birds in a large cage what do you think happens to their weight if they all become airborne? Their weight is now being supported by the air in the cabin instead of their perches which in turn is being supported by the floor of the aircraft. The weight of those birds is now evenly distributed across the floor of the aircraft just as the weight of a Jumbo Jet is evenly distributed across the surface of the entire earth when it is in flight. If this wasn’t the case, every time one flew over we would hear of the hoards of people being crushed to death by their enormous weight.
Another way to look at it is a swimming pool. The weight of the pool consists of the pool + water + swimmers. The weight of the swimmers is evenly distributed across the bottom of the pool not just below the spot they are swimming.
disco_air
Just like any object in the aircraft they will have their own centre of gravity. In the case of the airborne birds in the cabin their C of G will be roughly the centre of the cabin.