I'll say it again: the limit on flying bird-weight is simply the size of its wings, and that is roughly the calculation of 5 pounds per square foot of wing.
So, if a large Kori Bustard weighed 45 pounds and has a nine-foot wing-span, how wide do those wings have to be?
Answer: to lift 45 pounds you need nine square feet of wing (45 divided by 5 pounds per square foot).
Since Kori Bustard's wings are nine feet long, they need be only one foot wide -- as clearly they are:
varmintman: "The square/cube problem has done the same thing for the bustard, which can fly with difficulty for short distances, but that's it, i.e. it would be a mistake to call a bustard a flying bird."
But the fact that bustards don't fly much is a reflection of their life-styles, not some physical "law" which prevents them from it.
If these birds seriously needed to fly more, then somewhat longer, wider wings would accomplish that, as we can see in the case of ancient Argentavis, with max 26 foot wings lifting it's 176 lbs.
Those 26 foot wings needed to be less than 1.5 feet wide to provide the necessary 35 square feet of wing-lift.
The same calculation works for Quetzalcoatlus' 52 foot wing-span lifting perhaps 550 pounds -- for flight, those wings needed to be only two feet wide, which clearly they were far more.
varmintman: "The argument that bustards prove that a teratorn or a Texas pterosaur could fly in our world is idiotic."
Kori bustards, Albatross and Condors all prove that wing-loads less than 5 lbs. per square foot will get a large bird air-born.
All of the estimates for large extinct birds & pterosaurs also fall within that range.
varmintman: "The calculations Desmond mentions were perfectly good."
Such calculations are laughable, and the resulting conclusions ludicrous.