Sheesh, I wish I'd never come across this thread...
...it's kept me awake for three nights running.
That bird in the cabin again. So it's argued above that in an enclosed/pressurised cabin the bird, when it's flying, still contributes to the weight of the aircraft.
I'm still not sure that's true, but what about the other bit of my question - does it contribute to the weight if the cabin is not enclosed/pressurised (not sure which one of those is the significant one.) Clearly not, I'd suggest, as a moment's reflection on a bird sitting on your lap in a Tiger Moth will make obvious.
So what exactly is the difference when the cabin is enclosed/pressurised? I guess supporters of the bird-weight theory will say it's still the downdraft, but I have to say I found the response to my question as to what happens if the downdraft is artifically blown away by a fan unconvincing.
I also suspect that for the same reasons, some of you will say the Tiger Moth situation described above is not so simple. OK, but then the question arises as to how far does the bird have to go before it no longer contributes to the aircraft weight.
(Presumably this could in fact be tested on the ground - I wonder if it's ever been done?)