A truck driver, it is said, once explained that when he was crossing a bridge and was overweight with birds; he just rapped on the truck and got the birds flying, so the truck was lighter! Of course, it was explained on MythBusters that Newton's Third Law does not work that way; and by putting 36 pigeons in a truck and getting them to flutter up; it was shown that the weight of the truck (goes up and down a little) but, really does not change. However nothing was said about air pressure. I presume that by design most of the outside air came in and out at the top of the truck. Certainly, since the birds did not suffocate, there was a source of outside air. What if the bottom of the truck was made of open slats covered with wire mesh? Then the air pressure could escape out of the bottom of the truck. What then would happen to the weight of the truck as the birds took flight?