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Re: Ratite habitat preferences
On Sun, 26 Mar 2000, Tommy Tyrberg wrote:
> 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
Though the research is yet to be done, Brian Bertram believes that
ostriches avoid nesting in very productive areas because higher
concentrations of prey animals brings higher concentration of
predators. So savannah, yes, but in the more arid extremes, possibly.
> >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.
"especially arid" in Australia is indeed arid. The phrase "disturbance
infrequent and cover extensive" is taken (paraphrased?) from Davies 1964
(I think) study. This same phrase is parroted over and over again in all
the references. Again, I don't think the research has been
done. Extensive cover means what, and what is "coastal vegetation"? And
I would assume disturbance is an unfavorable factor for any nester. Also,
Australia is a strange case. Pastoral areas support highest
densities of emus. But they are new. What was there before
> 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'll check it out.
> I'm not sure whether it is
> worse or better on the ground,
Believe it's worse. I'll look for ref.
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.
Crypsis is critical. My hypothesis is that there is a limit on how big
an incubating parent can be and remain undiscovered.
> 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.
Flannery maintains there was a greater diversity of marsupial
carnivores on this continent.
> 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)
Yes. And I am not arguing that. The issue is strictly concealment: where
you can conceal you can nest (providing there are not too many predators
to stumble upon you).
. If nest
> predation was much heavier on the ground one would expect these
> ground-nesters to have changed to tree-nesting.
But there are many ways to make adjustments. Precociality, larger clutch
size (so more eggs survive), smaller clutch size (so you don't have to
make predator-alerting visits), multiple nesting attempts, change age
structure, i.e., have babies earlier, and so on. All of these have costs
> 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
I think the beloved hairy armadillo brings the above into question. It is
small but preys on eggs of large rhea. It builds a labyrinth of tunnels
neath nest. Nest collapses under weight of rhea which then abandons
nest. Also, ostriches are vulnerable to small animals (Egyptian
vultures, black-backed jackal).
> 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.
Undoubtedly! But they survive because they _can_ move them. I maintain
it is that long stationary period which is the major liability to egg
layers, i.e., it allows predators more time to find them. Also,
precociality in mammals is a very different thing than in birds. I mean,
inasmuch as organogenesis is extended regardless in mammals.
> 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
Well, this statement is in direct opposition of those made by Bertram in
his study of the ostrich mating system. He says that their entire
breeding system has evolved under the primary influence of nest and
> opportunities I've had to study them. They are a fascinating group of birds
> from several points of view.
And fromthe point of view that they must have a great deal in common with
non-avian dinosaurs, inasmuch as they were big, and hard to find