If the cargo hold is open to the outside, then, no the parakeets won't add weight. But if it's open to the outside, they also won't be traveling due to the speed of the plane. If it's simply not pressurized, but closed, that doesn't make any difference, as air isn't going to randomly move in or out of the plane's hold. The more open you make it, the less weight it will have to carry, but the fewer of the birds it will carry.