Just to clarify my post above, I agree with 404 Titan that the overall weight of an aircraft will not change, whether the freight is airborne or not. In my post I was trying to paint a word-picture of why I believe this to be so, without referring to Newton. But, I appear to have been totally unclear.
[New Year's Resolution - give up posting on PPRuNe...]
But there is one thing I'd like to ask - the weight of an aircraft does not change if the birds get airborne. Agreed. But what mechanism transfers the weight of the flapping birds, to the aircraft (which still has to support their weight)? Two of you have suggested that the downwash is irrelevant. However, please consider this chain of events:
The birds generate lift by accelerating air downwards when flapping. Accelerated air is brought to rest by friction with other air below the birds, so transferring momentum to air below the birds. Since rate of change of momentum = force, and weight is the relevant force here, can't we say that the layer of air under the birds is supporting the weight of the birds? The layer of air under the birds is itself supported by the aircraft, hence the aircraft supports the weight of the birds.
Regarding CG changes with moving birds inside cabin -
As far as lift and weight go, buoyancy makes a good example of what 404 is talking about. However, (and only because someone mentioned CG changes), buoyancy must not be confused with heavier-than-air lift in this discussion. Floating swimmers go to one end of the pool, or to the other, but the CG of the pool does not change, because the average density of every part of the pool remains unchanged.
However, the average density of different spaces inside an aircraft do change if birds move around (weight of air in forward cabin < weight of flapping birds in aft cabin), so would there not be some effect on CG if the airborne birds flew from one end of the cabin to the other?
Hoping I've explained myself better this time,
O8