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Re: Herbivore protection
At 02:19 PM 6/6/97 -0400, John Bois wrote:
>Stan Friesen says: "...colonial nesting *can* improve the
> This may be true (and thank you for the example). But it
>may not be true. For example, on what basis do they choose where
>to site their mass nests? Could they be places of relatively low
I doubt it. The main predators of pigeons would tend to be fairly evenly
spaced (see below).
> What were the predators?
Well, given we are talking small birds here, the bobcat springs immediately
to mind (remember "the cat among the pigeons"). And then there are various
snakes, which would be particularly dangerous to the chicks. And finally
there are falcons and accipiters (forest hawks), which would be major
predators on these birds, as they still are to their surviving kin.
Overall, I doubt there were any mainland areas that were particularly free
of these predators. And the passenger pigeon nesting grounds *were* in
mainland areas. They were physically easily accessible as well. That is
why they were so easy for us humans to hunt.
Yet, despite being small, defenseless, and prime meat for several potent
predators, including other birds, they were enormously successful until
humans decided their feathers were valuable. By all accounts they used to
be as abundant as starlings are today, or even more so. Our hunting them
to extinction is in some ways even more remarkable than our decimation of
the bison herds, since their intrinsic reproduction rate was higher.
>Also, some of your info differs from mine. For example, I read that these
>pigeons nest up in trees and not, as you say, in forest clearings.
Hmm, I could be wrong about that point, it has been awhile since I read the
article, and that wasn't the main point anyhow. Though whatever the
nesting sites were, they were vulnerable to human mass cultivation.
I cannot find the article right now. It is probably in my stack of old
issues of _Natural History_ (in Stephen Jay Gould's column therein).
> This limits some predators
None of the principle predators on pigeons would be particularly bothered
> and provides an immensely greater area
>to hide than ground cover. And P. pigeons did not nest in one "spot"
>unless you define a spot to be 300 sq. miles.
The nested in dense mass nesting colonies. Of that I am fairly certain, as
that *was* the point of the article. It is possible the colonies were miles
across - 300 sq. miles is only 17 miles on a side; large for a nesting
colony, but small compared to the total range of the bird, which was on the
order of several hundred thousand square miles. (Of course there were
several such nesting sites, but we are still talking concentrations of
about a thousand times their non-breeding density).
>But still, I don't think this example is fully applicable to
>colonial nesting dinosaurs. P. pigeons from Detroit fly to New
>Jersey. They leave all their fox, raccoon, squirrel, snake,
>predators behind. The only way a fox could make it to New Jersey
>in time would be to catch a bus. Migrating herbivorous dinosaurs
>could not have such a large skill differential between themselves
>and their predators as has a P. pigeon and a fox.
But a fox crossing the territory of a rival fox gets attacked, so even if
it could *theoretically* walk as far as the prey, it could not do so in
practice. And then there is the problem of the fox's altricial young,
which make it hard for the fox to travel far.
And the main predators of the pigeon, other than the bobcat, would have
been fellow birds, and thus just as capable of long distance flight. As
far as I can tell, however, the accipiters *didn't* particularly follow the
So the issue is clearly not differential *travel* *capability*, but
different social structures preventing the predators from fully *utilizing*
their ability to move about.
What would happen is that the herds of hadrosaurs would move with the
seasons to where food is most abundant, *crossing* the more localized, and
less easily shifted, territories of the individual predators and predator
packs. This is what happens in East Africa today. The lions do NOT follow
> I know long
>migrations are possible and they probably happened. But at least
>some of our modern predators move right along with the ungulate
If you are thinking of the prairie wolf, they were only a minor nuisance to
the bison herds. Also, I am not entirely sure it was ever really properly
verified that the packs *did* follow the herds beyond the home territory of
the pack. Wolf packs today tend to be *very* territorial, not just because
of rivalries, but also because wolf pups cannot travel for many weeks after
birth, binding the pack to the den site for that time. Separate
territories is certainly the way that lions interact with wildebeest and
zebra. I really doubt wolves could really manage things that much
differently, especially since they do not do so anywhere they still live
[A great lot of what most people think they know about bison is actually
just myth - for instance a herd almost certainly did NOT travel from Texas
to Canada: that is out of line with what we can determine about bison
biology from the few remaining animals].
>Stan Friesen doubts my claim that grunion exploit the high water
>mark because it is a place of low predator density. And he cites
>gulls, terns, etc., as predators of this zone. This is true. I
>meant predators of grunion _eggs_. ...
Hmm. Interesting. Maybe.
But there are more things burrowing about in a wild (tourist-free) beach
than you might imagine. This is not something I have really looked into.
Are there more mud burrowers in the sea bottom than sand burrowers on a
beach? I really do not know.
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