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Re: Pterosaurs and birds, was Re: birds and pterosaurs

In theory, evolution might bypass the problem in that
bird with extremely large wingspan has long flesh part
of hand, but flight-feathers remain at manageable

If you mean the arm itself is longer, then yes. But no birds have especially long projections of soft tissue at the distal portion of the wing. ie. the soft tissue does not form an appreciable part of the air foil.

I know about Argentavis etc. - it apparently existed
in ecosystem with unusually big density of food.
Judging that big pterosaurs are more common than
equally big birds, pterosaurs could grow bigger on
similar amount of food.

Are you thinking there is a metabolic difference then? There was a fairly high diversity of teratorn birds in the late Cenozoic, so it is not just Argentavis. Just the same, the presence of megafauna for scavenging purposes may have been a factor in the evolution of large teratorns. However, pterosaurs were probably primarily pelagic foragers, so the ecological analog is somewhat shaky.

It is not exactly physical constraints (body mass,
wing size etc.). It is ecological constraints
(like most biomechanical problems). Bird which
feathers grow for too long cannot fly "well enough to
feed". What is "well enough to feed" ultimately
depends from food in birds native ecosystem.

I am not really sure what you mean by this; I'm assuming that you're implying that birds of very large size would be poor flyers. However, large size is an advantage in soaring flight in many ways, and there isn't actually anything to suggest that teratorns or pseudodontorns would have been poor foragers. Take-off and landing would have been rather slow, but they probably did not need to do so often. Modern albatrosses are not as diverse as pelagic pterosaurs either, but they are extremely efficient soarers with great foraging ability. Besides, if teratorns and/or pseudodontorns could not forage efficiently at large size, it is unlikely that they would have evolved large sizes in the first place.

Judging that 7-m wingspan birds are sporadic in fossil
record, they need extremely food-rich conditions to
evolve. 7-m pterosaurs were more common - ergo they
were more efficent, they ecological niche was less

Actually, pseudodontorns were pretty common. Granted, there were more large pterosaurs for a longer period of time. However, I suspect the reason for the abundance of large pterosaurs was that most Cretaceous forms were marine soaring birds, which promotes the evolution of large size. Why there was such a diversity of marine soarers at the time (ie. Mesozoic, esp. the Cretaceous) is hard to say; pterosaur wing structure may have been well-adapted to that form of flight, or pelagic food resources may have been readily available (but see below). Alternatively, conditions may have been especially favorable for ocean soaring (shallow seaways with many marine thermals, advantageous prevailing wind conditions, etc). Certainly, pterosaurs were very efficient flyers, one way or the other. And yes, mechanical constraints do seem to differ such that pterosaurs generated more large-bodied flyers than birds, but this is not the same as being 'more efficient' or 'less restricted' (I suspect it was partially to do with the relatively lower wing loadings of pterosaurs per unit mass.)

One problem with the food-rich conditions hypothesis for large soarers is that while large soaring birds do need a large total allotment of food, they should also be better at covering large distances to find it. That is, a large soarer is actually well adapted to low food density (though total food biomass must be reasonably high).

PS. I am looking for more precise data about bird
moult - it seems that feathers grow more like
10mm/day, but full moult of vultures and pelicans
takes 3-4 years.

Thanks for the snippet; that's very interesting. I wonder why the molt is so slow in those groups...