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Re: [dinosaur] Evolution of giant flightless birds (free pdf)

> And yet plenty of volant species do quite well on poor quality plant fodder
> (e.g., geese).

True.  But would they do even better if they were larger?

All things being equal, they probably would do better. But all things are seldom equal. Large size brings better processing but makes it more difficult to hide. This is a critical issue for oviparous species. 
Here the decline of
dromornithids is attributed to competition with large diprotodontian
marsupials (who could achieve body masses that exceeded those of any
bird), and nothing to do with the birds being flightless.

I believe humans are still in the running for causing macrofaunal extinction in Australia. If so, large size has much to do with it. Also, the bloody dingoes may have played a role: for example, emu population sizes are much smaller on the dingo-side of the dingo-proof fence. And, as if to demonstrate the intensity of selection on the ability to conceal emu nest site, emu males incubate for over a month and never leave the nest. We know that activity around a concealed nest alerts predators. The emu goes without water and food for this entire time. To conserve water it pees a solid crystal. In any case, Worthy attributes the low diversity of Aussie big birds to the presence of carnivorous marsupials.