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Re: neornithean size
> One possibility that constrains size is related to flight. Above a
> certain mass, they may just not be able to generate enough power to lift
> off. Living ratites, of course, have escaped this problem by increasing
> total mass, reducing forearm size, and giving up flight.
It may be that for most birds, which are volant, acquiring, say,
elephant size is difficult because of aerodynamic reasons. Perhaps it
is unusual for not-flying birds to acquire this size because they are
very few (as the loss of flight may be a much minoritary event in bird
lineages compared with keeping airborne). There likely were much fewer
birds exceeding the 100 kg-500kg range size, than herbivorous mammals
in the same range size, and if only a small proportion of animals in a
given group is expected to increase in size in each group, then it
would seem reasonable that a small number of more than 100 kg bird
species is less likely to produce elephant-sized descent compared with
a larger number of mammalian herbivores in the same range size.
> There also may be metabolic limitations, eg ability to undergo sustained
> anaerobic energy production, but that has not been demonstrated AFAIK.
Their respiratory system seems to indicate they have advantage over
mammals relative to extracting oxygen in order to keep aerobic
metabolism. Hypothetical elephant-sized birds would have been at least
as efficient as gigantic dinosaurs in aerobic capacity, and likely
more, given that their air sacs excavate a largest share of their