The longer post of Itchybum explains it all with one slight remark... Compairing two different situations: one before takeoff of the birds, one after. For the birds to be able to takeoff they need energy. So they will have consumed energy which they got from... fat ie
. But also, the 747 will have lost some fat too
I have been looking for the "visual" proof, but can't find it. I guess the tv company removed it from their website. About a year and a half ago there was a very popular (despite being scientific) tv program on Belgian television where scientific questions were answered. One of them was: put a heli in a glass container. Seal it, then let the heli fly. Does the wait of the container (with air and heli inside) change when the heli starts flying? You should know the answer by now...
(ps: if you wondered, they used a small RC heli)