In article , Oz
writes
David G. Bell writes
On Monday, in article
"Oz" wrote:
In passing one seems to have spars seen regularly in a (small) area for
a couple of years and then they are gone, only returning some four or
five years later.
Yes, and it took a long time to persuade some ecologists that even very
simple mathematical models could show chaotic behaviour. That's now
fairly well-understood, and short-term population crashes are not viewed
with the same professional alarm. The trouble is that it's hard to
distinguish the chaotic pattern from some real disaster, and, since
birds are so mobile, breeding populations are hard to localise.
Still, there is a difference between local boom-bust patterns and the
larger-scale population. If Oz sees a population crash, with no
apparent reason, it'd not a big worry. If a lot of other people see the
same...
The reason is only too apparent:
the spar has run out of small birds in its area.
As noticed by visitors to birdtable.
Incidentally they have *never* been as numerous as they were before
spars appeared. This is despite siting birdtable close to cover and
under branches, severely impeding spar attack.
Damned squirrels, though ...
In that sense, I'm fortunate in living where there are no squirrels!
However, I have been feeding between 50 and 80 sparrows throughout the
winter, plus 30 or so chaffinches and assorted other birds, including
starlings and collared doves, all of which have in turn been helping to
feed a pair of sparrowhawks. Numbers of both sparrows and chaffinches
have been higher this winter than last, presumably because of a good
breeding season last year.
--
Malcolm