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Re: Nocturnal crocs?




On Sun, 26 Aug 2001, Tommy Tyrberg wrote:

> Out of eleven extant ratites (two ostriches, two rheas, one emu, three
> cassowaries and (at least) three kiwis) there is exactly one exclusively
> grassland/steppe species, the Lesser Rhea. The ostriches occur in almost
> any habitat except rainforest and (waterless) desert and the same is true
> for emu. 

This discussion relates only to _nesting_ habitat.  Ostriches nest at
highest densities in grasslands (Bertram).  Ostriches probably avoid treed
areas because of the cover this provides stalking predators
(Bertram).  Good vegetation years may be bad for ostriches because they
bring higher predator density (Bertram).
I'm not saying anything revolutionary here--just that the success of nests
depends upon their likelihood of being discovered.  If ostrich nests are
discovered, they are destroyed (Bertram). 
Small ratites are irrelevant to the discussion.  Emu depends upon mixed
vegetation for cover, including grass.
Cassowaries are indeed forest birds.  Why are they the biggest birds to
nest in forest?  This is a fair question.  Are there reasons of predator
density, predator quality, biogeographical history?  I would love to
kmnow.  In any case, this particular strategy--forest nesting for big
birds--is no sweepstakes winner.


> >Gastornithids existed while NA and Europe were covered with wetlands.
> When each continent drained, they >disappeared--thopugh the draining
> occurred at different times on each continent.
> 
> Really? Wetlands all the way from western North America to Europe? Also
> wetlands is just about the only habitat that is _not_  used by any extant
> ratite.

Increased wetland habitat in Eocene.  Andors (ref. if needed) cites
studies showing more humid/wetland/swamp/whatever habitat in both Eu and
NA.  When these continents switched to drier climates, Gastornithids
(Diatryma back in 1988) fossils disappear.
Extant ratites exploit vast _grasslands_.  Wetlands do not exsit in past
vastness.  Look, this is an hypothesis.  I don't know if it is true.  It
_is_ a fact, however, that wetlands are at least a partial refuge from
mammals.