Passenger pigeons and Carolina Parakeets:
Once there was a North American bird called the passenger pigeon.  My father spoke of them with a kind of regret.  He said the flocks were so dense that a hunter shooting into a flock was pretty sure to bring down a bird.  According to the Wikipedia, flocks could be a hundred miles wide and hundreds of miles long in migration.  They must have darkened the sun for hours.

Then they all died out.

If indeed the birds were so abundant and mating among the members of a flock were random, then they would be a serious challenge to what I maintain, which is that large populations cannot remain fertile.  The birds were reported to be in large flock over a span of centuries.  In fact, as they started to die out, a effort was made to raise them in captivity.  This effort failed.  The conclusion was that they could only mate if they were in a huge flock. 

I think this is an example rather than a counterexample to the principle.  The enormous numbers clearly existed.  And I have never heard of anybody who thought they could distinguish one bird from another.  But that is not to say that the birds could not tell.  A penguin can pick out her own offspring from a large group of pretty much identical birds after a long absence.  Pigeons probably can, too.  And the large flocks were not constant.  They lived in smaller flocks most of the time, only teaming up when it was time to migrate.  They had every opportunity to get to know each other and to stick together even though the human eye saw only an enormous mass.

So why did they die out?  There is plenty of evidence for massive hunting and environmental disruption, but it seems hard to think that was sufficient in and of itself.  I suspect that these elements simply served to break up the little flocks, to disperse them so that members joined other flocks, possibly after a migration.  Thus churning up the gene pool could have done it.

And of course people found it hard to raise them in captivity. No such effort would have been made until the population was in freefall.  By then the birds were already infertile.  Caged, they could only live out their span much as they would have in the wild. 

There was another notable North American bird that died out: the Carolina parakeet.  It was nothing like as numerous as the passenger pigeon, but they still existed in large numbers.  They were not as tasty as pigeons, so hunting was no great problem.  Nor were they killed as pests.  In Florida, where they made their last stand, they were valued because they ate cockleburs.  Yet they vanished.  I expect again it was environmental disruption that broke up their mating pattern and left them mating at random until their fertility fell to nothing. 

M. Linton Herbert MD 

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