Re: Does this moa argument fly?

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

On Sun, 13 Sep 1998, B. Choo wrote:

> The NZ pre-human avifauna, while not threatened by any terrestrial predator
> (pack-hunting Tuataras anyone?), still had to contend with attack from the
> air (Haast's Eagle, Giant Harriers and Keas), why would the absence of
> mammalian predators alone be a factor in the face of avian attack?

I agree that aerial attack was likely a problem for these species.  It is
with the Rhea and the Ostrich.  However, I believe predation _sums_.  In
other words, the more predation the worse off a species will be.  Perhaps
mammals and birds give a one/two punch which, where they co-occur, all but
eliminates a difficult-to-hide nester.  for example, birds tend to take
juvenile prey while mammals may take eggs and juvenile prey (with the
amazing exception of the Egyptian vulture).  In this way the presence
of mammals consists of a whole nother threat.
In addition, I believe the moa may have had better protection from
overhead birds because they lived amid vegetation of greater structural
complexity than. for example, rhea or ostrich.  As such they would negate
the value of wide area searching as seen in many birds (speculation).

> The Mihirungs (Dromornithidae) of Australia, some of which may have
> surpassed the 500 kg mark, coexisted with both large mammalian herbivores
> and terrestrial (mammalian and reptilian) predators.

Yes.  But some of these were open grassland creatures and so
could use space to hide (much as emus, ostriches, and rhea do today).
This niche was largely absent in NZ. 
Also, I am trying to gather an argument which says marsupials, even
carnivorous marsupials, were not as good at finding things as placentals.
In this sense, Australia may have always been a kinder, gentler country
within which to incubate your children.
Thank you for your comments,
John Bois.