As for the birds.... flying might be one thing, but what if they jump (no flapping, so no transfer of force from legs to air movement)? Like the group of footballers sitting on the back of a bus all jumping as it goes over a speed bump. I can testify that the bus does indeed behave as if it's half a ton lighter all of a sudden!
As for the other questions:
1) In terms of air velocity, yes... the local velocity will be a long way under supersonic. However, most of the air will be sucked into the slipstream, and the engines tend to be a little loud
2) Yes. But it's all relative
3) No. The load allowance for catering is considered part of the ZFW regardless of how much has been eaten. Anything that is excreted ends up sitting in the tanks anyway... So, not possible scientifically, or legally.
I think
Lancer