"Tumbleweed" wrote in message
. ..

"W K" wrote in message
...

"Oz" wrote in message
news
Tumbleweed writes
ISTR reading a few years ago that the number of sparrow hawks had
risen
hugely since the 1940's, maybe to 50,000 or more, whereas 60 years
ago
or
so
they were persecuted and very few and far between. If the difference
was
say, 40,000 sparrowhawks between then and now, and they each ate 1
sparrow a
day, that would be 14 million less sparrows a year. Plus, every day I
see
loads of magpies (probably 10 or 20), which I believe eat other birds
eggs.
When I was a kid I don't think I ever saw one. I would guess the
number
of
magpies must have risen 10 fold in the last 30-40 years. That must
account
for a fair few sparrows (and similar) as well. Certainly there are
lots
of
aphids and the like in my garden in the summer and I would have said
that
most gardeners nowadays used less chemicals than 30 years ago. You
certainly
cant have a rise in the number of predators and expect the prey to
remain
constant,

You can, if the predation is not the main thing controlling the prey
numbers.
If the limiting factor is breeding sites and food sources in winter, the
ones eaten will just leave more resources for the others and the population
level stays precisely the same.

after all isn't that the point of all this organic gardening
we
hear about, encouraging predators such as hoverfly and ladybirds into
gardens? If that works for them, I don't see why it wouldn't work for
sparrow hawks/ sparrows as well.

How do you do that encouragement?
Are you bringing/attracting these creatures in from elsewhere?
I believe one trick would be to attract more insects in general so that
there is a wide selection of prey for them.
It would certainly work on a local level if you set up your garden to be an
ideal killing zone for the sparrowhawk - but that would probably not effect
the overall populations averaged out over the sparrowhawk's range.


An equivalent might be to feed a predator and keep its level at higher
levels (ie domestic cats, corvids given sources of carrion that help them
get through winter in greater numbers)
This is nothing like the case with the sparrowhawks (although perhaps all
those well fed tits could make a minor difference through the winter).

"I don't see" Glad to see such an admission of cluenessness


OK, so if you're so clued up, give us the benefit of your all-knowing
wisdom, or do you just do poor sarcasm?

With that level of radio2 lunchtime ramblings I'd have thought it wasn't
worth it.
Its about reaching levels of year-round semi-equilibrium, and what the
limiting factors are.
Sparrowhawks are unlikely to overexploit their area and cause population
crashes. (and you know, they eat lots of birds that are having no problems
at all, or increasing).