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Re: Waiting for a giant bird and dino physiology revisted




On Sun, 13 May 2001, Ronald Orenstein wrote:

> >  And, juveniles below
> >some size would be immune from predation, I would think.
> 
> Why?  This is quite the opposite of most predator-prey situations, and 
> large predators will certainly take small prey if it is relatively easy (eg 
> energy-efficient) to catch.  Besides, NZ still has a falcon and a harrier 
> that might have once been able to take chicks of smaller moas (not to 
> mention a fossil crow that may have done the same).

It is true that such birds are terrible predators of extant ratites'
babies.  But they are not the main cause of nest failure due to
predation.  Mammals are.  And the moa didn't have to worry about
mammals.  The point of all this was David's claim that there are no really
big birds because large size is not selected for.  I responded by saying
selection for large size is trumped by selection for small size where
predation is significant.  I also said I couldn't imagine what selection
for large size existed in NZ that was absent from most everywhere
else--that it was more likely absence of (mammalian) predation that
relaxed size constraints, allowing the influence of milder selection
pressures to kick in (e.g., economies of scale, etc.).  So, I agree that
juveniles were not "immune", but they were probably relatively safe
compared to continental chicks.

> that selection _for_ large size, and selection _against_ large size is at
> >equilibrium in the ostrich: further increase in size would
> >both slow it down and increase its visibility at the nest; a decrease in
> >size would also slow it down (I'm guessing).
> 
> This makes rheas a bit hard to explain! And dromornithids were a lot bigger 
> than ostriches, and faced thylacines, marsupial lions, sebecid crocodiles, 
> giant monitors etc.

I believe ostriches are faster than rhea and they have to deal with faster
predators.  No one has timed marsupial lions...they may well have been
significantly slower than the continental cats.  And, I don't believe it
is known whether or not any of these creatures preyed on dromornithids--if
they did, dromornithids may well have had as effective a strategy as
today's emus--that is, they hide very well at their most susceptible
time--egg laying.  At least it is known that Genyornis laid her eggs in a
similar environment as emus (their eggs are found together!).  As you
know, with ostriches, concealment is critical--found eggs are
destroyed.  It is possible that Genyornis was able to hide in a similar
manner to emu.  It is also possible that Australia supported a lower
density (not diversity) of carnivores, and/or that the marsupials were not
as clever at finding nests.

Finally, since it is apparent that _all_ ratites go to great pains to
conceal their nests; and that all nests found are destroyed; then it makes
sense to claim there is a theoretical size limit beyond which concealment
is not a viable strategy.  This is at least  a possible contributing
factor in keeping birds small.