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Re: a closer look at Hanson 2006, Mortimer response
The smallest known adult birds are hummingbirds as far as I know, and
those require at least 20 days or more before they are strong enough
to fly. That's not 'volant from the time they leave the egg'.
I believe David is actually referring to the analysis of a pterosaur
embryo which suggested that the animal would have hatched in a
superprecocial state (ie. the embryo had well-developed wings, etc). I
don't think he was making the 'volant from birth' argument based purely
on body size. Please correct me if I am wrong, David.
Regardless, however, I agree that the argument that pterosaur offspring
were volant at birth to be weak. Superprecocial birds are not volant
at birth, though they do attain flight at sizes below adult body size
(which is rare amongst birds, but may have been common among
pterosaurs). The presence of adult-like proportions does not in and of
itself strongly suggest volancy at birth. It does suggest early
volancy, but no more than that.
For one thing, in order for pterosaurs to be volant at birth, they
would need to have eliminated the loading component of bone geometry
determination, and derived an entirely genetically-determined long bone
geometry. I find this unlikely at present.