Have you lot not seen a pigeon carrying truck? I know the question is hypothetical, (maybe assuming that the cages are airtight) in which case the weight is constant, but they are pigeons! so it doesn't really work very well as a model, if you bring common sense into it the pressure created by the flapping wings (read as weight on the scales once birds are a'flappin) which would have to be equal or greater the the weight of the bird only effects an overall weight if all pressure is directed downwards, which of course it isn't, as the pressure outside the lorry (through the cage type thing) is less than the pressure created by the downward pressure of the wings innit. The argument here is hypothetical v's reality. Buy of of these, fill it with pigeons and put it on a weigh bridge and see if i'm wrong. You could do it on a smaller scale with a budgie in a cage and some kitchen scales - anyone game? 
