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Harmonious depredations



I said:
>>If someone discovered big terrestrial egg layers and mammals
>>(particularly placentals) living in harmony--without having to hide, I
>>would run away forever.

Ron Orenstein replied:
>As it has been repeatedly pointed out that this is indeed the case for
>ostriches and emus, both of which coexist with potential nest predators
>and often nest in open, completely unconcealed situations, I am inclined
>to doubt this promise.

Martin Human added about Canada geese in suburban Chicago (?):
>the geese in these exposed sub-optimal sites are now leading broods of
>goslings around; clearly they are not being predated, as John's idea would
>have it.

And Stan Friesan said
>Having seen film footage of ostrich nests, I concur with this
>characterization - the nests can be quite exposed indeed.  The female,
>however, guards it *violently*.

Anecdotal evidence is often misleading.  Indeed, in this case, anecdotal
evidence almost has to be misleading.  Documentary cameras must be able to
find their subjects and film them without too much getting in the way.
Where do they go to do this?  To places where ostriches nest in the open.
On the other hand, Brian Bertram(1992) did a scientific study in an area
enjoying a high density of nesting ostriches.  "Just finding nests was the
greatest problem of the whole study."and "(Ostrich nests are) invisible to
the terrestrial observer except within perhaps 10m because of concealing
ground vegetation".

Martin's observation is remarkable; but it is remarkable because it is so
rare.  In my fifty years of existence, I have never seen geese-size birds
nesting in a visible suburban location.  Doubtless this is an aberration
caused by human influence: leash laws, predator extirpation, and so on.

What about Ron's claim that ostriches nest in harmony with predators?
Bertram (1992) addresses this:

"Semi-arid, open, short grass plain is usually associated with the highest
ostrich densities.  Highly productive and very well vegetated areas usually
support high populations of competing herbivores, and therefore high
predator populations.  Adult ostriches are vulnerable to predation
particularly by lions (Panthera leo) if there is a great deal of cover.  And
ostrich nests are vulnerable to nest predators such as hyaenas (Crocuta and
Hyaena) for about 2 months; a very high hyaena density in an area could
result in almost all ostrich nests being discovered and destroyed before
hatching takes place.
        At the other extreme, ostriches are able to thrive in very poorly
vegetated areas, and can be found in dry savannah to desert.  They exhibit
a number of adaptations for dealing with the harsh desert
conditions: extreme temperatures, a lack of water and little food."

A reasonable inference from this is that in the extreme arid limits of
ostrich range, predator density is low.

Stan's observation of vigorous nest defense is interesting.  A reasonable
accommodation would be: Ostriches position there nest to avoid predator
contact, but may, upon discovery, defend it vigorously.  However, as Bertram
(1992) notes: this defense does not extend to the night hours.  Black-backed
jackals repulsed in the day time, return at night to worry the bird off the
nest and destroy its clutch (1992).

Finally, the concept of "core nesting habitat" is a key to understanding
this issue.  Populations boom and bust in response to various factors.  But
nesting species must have an area that, even in the hardest of times, can
support a sizable breeding population.  I suggest that the semi-arid
grassland is, for ostriches, this area.  It serves the ostrich both in
production (of food) and reproduction.  And concealment must be considered a
key attribute.  That ostrich evolution was apparently coincident with the
evolution of the grassland biome--not the desert biome--is further
evidence of the importance of grass to this species.

Bertram, B.C.R  1992.  The Ostrich Communal Nesting System.  Princeton
University Press, Princeton, NJ.