Re: Ratite habitat preferences

[Tommy Tyrberg <tommy.tyrberg@norrkoping.mail.telia.com>]

At 12:19 2000-03-26 -0500, John Bois wrote:
>
>
>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...
>
Ostriches certainly occur in very dry desert but I doubt that this is their
core habitat. My impression is that densities are highest in savannah
habit, though this might be due to heavier human hunting pressure in desert
where they are easier to locate.

>> 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?

No, I cite Handbook of Australian, New Zealand and Antarctic birds:  "in NW
Vic. aerial surveys found Emus more abundant in Woodland than in mallee....
in inland WA... most common in areas of bowgida bush Acacia linophylla and
tending to avoid Spinifex Triodia [grass]......Breed throughout range but
tend to be confined to areas where disturbance infrequent and cover
extensive...Little or no breeding in especially arid districts except after
heavy rain. Usually most common i s. coastal lands..... Density always low
in n. woodlands and other dry areas....only occur in farmland where patches
of woodland or coastal vegetation remain"

>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.
>
Certainly, it would be very odd otherwise wouldn't it?

>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?
>
This whole matter of predation pressures is rather complex. Nest predation
is *extremely* heavy in rainforest trees (David Snow covers this well in
The Web of Adaptation, which I recommend).  I'm not sure whether it is
worse or better on the ground, but there are many ground-nesting rainforest
birds (e. g. tinamous, cracids, megapodes, pheasants, francolins,
junglefowl, rails, pittas, tapaculos), so it's certainly not prohibitive.
There are actually rather few large ground predators in rainforest, mostly
probably because there are few ground-living animals of respectable size to
hunt. I do agree that predation (both on the ground and in trees) is
probably lighter in Australasia than in the other continents.   However the
same was probably true of predation on the ground (but *not* in trees) in
South America up to the Great American Interchange.

There are fair numbers of ground-nesting birds in all wooded/forested
habitats I can think of, so ground-nesting is clearly a viable option (in
treeless habitat all species are of course ground nesters). If nest
predation was much heavier on the ground one would expect these
ground-nesters to have changed to tree-nesting. The potential for this
certainly exist, many predominantly ground-nesting groups have the odd
tree-nesting species (ducks, waders, gulls, terns, auks). There are
certainly problems with tree-nesting for species with precocial young but
these are not insurmountable (as proved e. g. the Goldeneye). Also would
*large* species really be more vulnerable to nest predation? Their nests
are more conspicuous, true, but on the other hand they would probably be
able to defend the nest from small predators, which are after all most
common. Also I'm not sure that nest-predation is specific problem for
egg-layers, most mammals are also affected, except those with young that
are precocial enough to follow their mother immediately after birth and
marsupials. For example Swedish radio-tagging studies have shown that
Pine-martens move their young every 24 hours, apparently for good reasons. 

In short I don't think that predation on ground nests in the Cainozoic is
so fundamentally different in intensity that it would have a determining
influence on evolution, and in

>Thank you for your input.  And may I ask in what context you work with
>ratites.

I don't work with ratites in particular. My main field of research is the
Neogene and Pleistiocene evolution of avifaunas. I've just used any
opportunities I've had to study them. They are a fascinating group of birds
from several points of view.

>Cheers,
>John Bois.
>
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