[varmintman]

http://www.krugerpark.co.za/africa_kori_bustard.html

"They are ground dwellers, hence the name bustard, meaning birds that walk. They fly only when necessary because of their weight. It is even appearing that the Kori Bustards may become categorized as the few large flightless birds like ostriches and emus, which means they may be returning to an ancient ancestral form, since they, and the other cranes, are descendants of large flightless predators."

Basically, Bustards fly about as well as chickens do and are evidence that Adrian Desmond's note ("Hot blooded dinosaurs") about calculations as to flying maximums was correct:

"It would be a grave understatement to say that, as a flying creature, Pteranodon was large. Indeed, there were sound reasons for believing that it was the largest animal that ever could become airborne. With each increase in size, and therefore also weight, a flying animal needs a concomitant increase in power (to beat the wings in a flapper and to hold and maneuver them in a glider), but power is supplied by muscles which themselves add still more weight to the structure.-- The larger a flyer becomes the disproportionately weightier it grows by the addition of its own power supply. There comes a point when the weight is just too great to permit the machine to remain airborne. Calculations bearing on size and power suggested that the maximum weight that a flying vertebrate can attain is about 50 lb.: Pteranodon and its slightly larger but lesser known Jordanian ally Titanopteryx were therefore thought to be the largest flying animals."

In the case of the chicken, you have a member of the same family as pheasants which started off as a 1.5 lb. jungle fowl and was then bred into a 6 lb. meat animal but still has only the 1.5 lb bird's wings. 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.

The argument that bustards prove that a teratorn or a Texas pterosaur could fly in our world is idiotic.

The calculations Desmond mentions were perfectly good. Large pterosaurs could not possibly fly in our present world but they did in fact fly (you can't live by dragging 50' wings around). The only reasonable and rational analysis is that they did not experience gravity as we do.

[BroJoeK]

varmintman: "Calculations bearing on size and power suggested that the maximum weight that a flying vertebrate can attain is about 50 lb."

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.