"BAC" wrote in message
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"Nick Maclaren" wrote in message
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In article ,
Malcolm writes:
|
| If the birds weren't shot, the mortality would go down and their
| life-expectancy up.

That is extremely unclear, and might be the converse of the truth. It
will depend very much on how close they are to overpopulation at the
stressful times of year.

It is quite possible that stopping shooting them would cause an increase
in their population, and a consequent increase in their mortality rate
and a reduction in their life expectancy. That is what often happens
to prey species with effectively no predation - as is the case for
wood pigeons in many parts of the UK.


Are you saying that, for a given ecosystem, only a certain maximum number
of
a population of birds can be expected to survive the winter, regardless of
how many more than that number started the winter?


All other things being equal any given ecosysyem can only produce
a limited amount of food which can only support a limited number of
birds.

Given the rate at which birds such as wood pidgeons can reproduce
its fairy clear that all other things being equal they will exhaust
the food resources of any given ecosystem within a few generations.
With surplus birds either moving elsewhere - which doesn't really
soleve anything, or simply dying from starvation. Thus maintaining
an equilibrium.

The same probably applies to song birds as well. The large numbers
which are allegedly killed by domestic cats each year, quite possibly
free up food supplies over winter for others which would otherwise
have perished. It's simply that birds killed by cats are probably more
noticeable than those which starve in out of the way places
and rot down to nothing within weeks.


michael adams

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