Flying Payload

Question: Suppose you had a plane with a 2000 lb payload (it could not fly 
if carrying over 2000 lbs) and you stuffed 2000 pounds of birds into the 
plane.  If the birds began flying in the plane, could the plane now take off?
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The question really is, is the total weight of the plane changed 
by the activities of those birds flying around inside it?  The answer is that 
it cannot be.  The birds are supporting their weight in the air by flapping 
their wings, certainly, and this increases the air pressure on the ground, but 
the total force on the floor of the plane is the same whether the birds are 
sitting on the floor or flying steadily around in it.  Now, if the birds were 
not actually flying, but were jumping up and down then the force on the floor 
might fluctuate - if the birds were all coordinated the force could be quite 
large for a short while, and then very small until they hit the floor again.  
Over time the weight would still average out to the same 2000 pounds, though, 
and the plane should be able to take off (maybe with a few bumps and jumps).

A. Smith
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I completely agree with the first response, but would like to add 
the following twist.  Imagine this is not a pressurized plane.  In fact all 
the windows are open and the atmosphere in the plane mixes freely with the 
outside.  Now the birds flying does indeed decrease the weight of the plane.
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