I havent read the article but
IMHO
Force (Weight) = Mass X Acceleration
Acceleration = 9.81 (a constant on Earth)
The weight of the Aircraft includes the
weight of the air contained in it
e.g The weight of 22.4 litres of Nitrogen at STP is 14 grammes (this is 71 percent of air)
Add in the oxygen etc and its say 20 grammes
for the same volume
Incidentally:
The vertical component of weight for this gas is greater then the outside atmosphere at cruising altitude due to pressurisation
The vertical component of lift for the birds
is equal to their weight
The weight is supported on the molecules
of Nitrogen etc in the cabin. The downwards component of molecules colliding is represented as pressure on the bottom surface of the cabin (like the Tornado)
Therefore
The weight is the same as it was when it
took off is it not ?