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Re: neornithean size
Roland Sookias <firstname.lastname@example.org> wrote:
> It's an interesting question and something I've pondered a lot.
> Mammals are another factor - both outcompetition and ovivory. Places
> where birds seem to have got big (e.g. New Zealand, Madagascar) had
> limited mammalian faunas.
Yes, it's a very interesting question, and like Augusto and Roland
I've wondered about this too.
Although the proliferation of ratite species in New Zealand was no
doubt influenced by the absence of terrestrial mammals, I'm not wholly
convinced that mammals necessarily constrained the evolution of large,
flightless birds in general. The Dromornithidae (which arguably
includes the largest birds ever) lived during a time when there was a
much more diverse mammalian (marsupial) fauna in Australia than there
is today, including predators. The North African _Eremopezus_ shared
its habitat with a variety of placental mammals, large (or large-ish)
herbivores and carnivores among them.
I tend to agree with Alan that there are potential physiological or
metabolic limitations to birds becoming utterly huge. It may have
something to do with the need for such a large animal to sustain a
high avian-level metabolism, and shed surplus heat as the ratio of
surface area to volume declines. I have no data to back this up -
it's purely intuitive and speculative.