Ratite habitat preferences

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

On Sat, 25 Mar 2000, Tommy Tyrberg wrote:

> Since I happen to have field experience of all extant ratites (except Dwarf
> Cassowary) I know that this is simply not true. A quick summary of habitat
> preferences:
 
> Ostrich: avoids closed forest, otherwise almost ubiquitous. I've seen it
> (breeding) in e. g. mopane woodland, dense acacia woodland, fairly extreme
> desert and coastal fynbos.

I think it's fair to bring in the concept of a core habitat.  Animals
adapt, or have advantage in some habitats over others.  My impression of
ostriches (having only read about them ) is that they are arid habitat
specialists.  They tolerate very dry conditions; are adapted to these
conditions; can survive in them where other animals cannot.  I'm sure
you're not denying that concealment is important for nesting, nor that
some habitats afford better concealment than others.  From Bertram, I
assume the core nesting habitat of ostriches is in dry grassland.  The
ostrich nests in a territory of 2km. sq. and can only be seen as close as
10 m.  Predators are few due to low productivity in arid region.  I'm not
saying they cannot nest anywhere else.  I _am_ saying that without this
core habitat (I'm also assuming this habitat supports the highest nesting
density) ostriches probably would suffer loss of recruitment, and maybe
become extinct.  Similarly, prime nesting habitat loses for waterbirds
brought about significant population drops.  They can still nest in some
suburban backyards, and so on, but over time...

> Lesser Rhea: puna, steppe and scrub steppe
> 
> Greater Rhea: grassland (campo), cerrado, chaco woodland

Predator extirpation has no doubt relaxed limits on rhea nesting
habitat.  But the habitats you list above support my claim, right?

> Southern Cassowary: Lowland rainforest, in New Guinea also in swamp forest
> and savanna woodland
> Northern Cassowary: Lowland rainforest, swamp forest
> Dwarf Cassowary: Highland rainforest

Can I say the Cassowary are the exception that proves the rule?  If not, I
have to invoke low predator density.  Tim Flannery, at least, says these
areas have historically supported a depauperate predator population.
 
> Emu: not in rainforest or completely waterless desert but otherwise almost
> ubiquitous, including closed forest. 

Prime habitat is arid savannah--true?
Also, two emu experiments support me.  Population counts either side of
dingo fence show great reduction on dingo side.  Study in an island nature
preserve suggest predation is entirely on eggs and chicks.

As you know, predation on adult ostriches is rare.

> Brown Kiwi, Lesser Spotted Kiwi, Greated Spotted Kiwi: All forest birds,
> though the Brown Kiwi does venture into open habitat to some extent.

Small bird, not relevant.
 
> Thus, ratites occur in almost all habitats except taiga and tundra (though
> ostriches did live in steppe-tundra in Mongolia and Siberia up till ca
> 10,000 years ago). I suppose You might consider Australia to have "low
> predator density", but this was hardly true 50,000 years ago with
> Thylacinus, Thylacoleo and Meiolania, and it's hardly true today with
> humans, dingos, cats, foxes and wild pigs. Actually Cassowaries and
> especially Emus survive these a great deal better than do most australian
> mammals, marsupials *or* placentals. 

I'm not saying ratites are inferior strategists, nor that they can't
survive in presence of predators.  But, it is a hard fact of ratite life
that if their nest is found, it is destroyed.  Two factors play into
this: predator kind and their density; and the quality of concealment.  My
position is that in ostriches, rhea and emu, the core habitat, supporting
highest populations, is grassland because it provides good cover at low
predator density.  Is this a reasonable position/hypothesis?

Thank you for your input.  And may I ask in what context you work with
ratites.
Cheers,
John Bois.