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This is off the top of my head, but it should give you an idea of what a bottleneck is...
 
A bottleneck is generally used in reference to a population's size over time.  Imagine a population starting with thousands of individuals and staying at that size for decades.  Then, abruptly or not, the population declines to a very low number, say just 50 individuals or so.  After some time the population expands again to a few thousand individuals.  This population has experienced a bottleneck, or a constriction in the population size.  
 
The length of time spent at low population size equates to the severity of the bottleneck.  Populations maintained at low population size for many years generally don't fair as well as populations that quickly rebound to pre-bottleneck population size.  A side note, the textbook example of a species that has undergone a bottleneck is the African Cheetah.  So much genetic diversity was lost during a population bottleneck that cheetahs today are more similar (in terms of their genetics) than lab mice.  Scientists have successfully performed skin grafts from one cheetah to another, unrelated cheetah.
 
Examples of avian species that have experienced bottlenecks in the US include the Bald Eagle and the Peregrine Falcon.  Populations of these species have increased and are generally considered stable, thus they have passed the bottleneck.
 
Populations still in a bottleneck (because the population size is still very small) include the California Condor and presumably the Ivory-billed Woodpecker.
 
Regards,
Chadwick Rittenhouse
Ashland, Boone County, MO
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Jean Leonatti <[log in to unmask]> wrote:
Thanks for sending this - a very optimistic note.

In several articles, I have seen the reference to "bottleneck" but no one
has defined it (at least that I have seen). Does anyone know what this is a
reference to? Is it everything being squeezed into the few remaining intact
forests? Or is it a reference to the forests being squeezed off from each
other so the "connection" allowing species to move from one forest to
another is so thin? Just curious - not looking for a scientific
dissertation, just layman's explanation.
Jean Leonatti
Boone County
Columbia, MO
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