Myth Busters: Birds in a Truck Education Q?

Question

How come the truck weight is no different when birds in the truck flies all together?

Answer 1

The MythBusters theorized that the air being displaced by the birds' wings (and the helicopter rotors) was pressing down on the truck, which is why there was no change in the overall weight. All was in compliance with Newton's third law of motion.

Namely for every action, there is an equal and opposite reaction.  The action of the birds flying displaces the air in a contained system and the force downward (expended to keep them aloft) is equal to their mass when they are not in flight.