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

John Bois <mjohn.bois@gmail.com> wrote:

> 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?  A positive
correlation between body mass and herbivory has been observed in
terrestrial vertebrates; but the cause is not settled.  Does large
body size promote a shift to herbivory, or does herbivory promote
larger body size?  According to the first hypothesis, large body mass
favors herbivory because it drive a shift toward an abundant and
easily accessible food source, such as leaves of plants.

The second hypothesis posits that increased body mass is advantageous
for the use of low-quality diets due to improved digestive efficiency.
This hypothesis has been invoked for the large size of dromornithids
in Australia (e.g., "Selective pressure for increased body size was
extremely high, as it increased the energetic efficiency in processing
coarse browse that was becoming increasingly prevalent with the
dominance of more arid-adapted (xeromorphic) scleromorphs in the late
Miocene." - from _Magnificent Mihirungs_.)  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.