Dromornithids and size limits.

[John Bois <jbois@umd5.umd.edu>]

Reading "Magnificent Mihirungs...".  Murray and Vickers-Rich make what is
to me an astonishing claim: there is an upper limit to how big birds can
be.  That claim seems to be based upon the maximum size of known fossil
species.  The authors suppose that egg size is the reason--that, to be a
very big bird, the egg has to scale along with you, and that the max size
of eggs is limited by (I suppose) gas exchange requirements.  Now, I don't
have the entire book, and so, they may go into it further--but based upon
what I have read there I question why they haven't considered alternative
hypotheses, for example, there is no selective value above Elephant
bird-size.

Indeed, egg size does not have to scale with body size--so, it is
realistic to propose a bigger bird with a smaller egg.

In any case, they use this questionable logic to support their claim that
these birds were outcompeted by large marsupials because, in part, the
latter species had no such restrictions on their maximum.

What am I missing?