A comment to Philip Veerman's recent
posting on House Sparrows:
Interesting story. But I would be very
cautious in linking events in Europe and with those in
Canberra. Even in
Europe,
House Sparrow (HS) decline is not universal. For example, in the German city of
Hamburg the population
has declined significantly, while in Berlin it has not
(based on long-term data sets published recently (Witt 2005) in a German bird
journal).
I also have difficulty linking declining numbers
of HS to an increase in the numbers of Sparrowhawks. While both events may be
real, I doubt that there is a simple cause-and effect link. To me it points to
complex interactions of many variables affecting both species simultaneously,
albeit in opposite ways. Intuitively, I would say a decline of HS numbers should
not lead to an increase in the predator, assuming the HS is a key
prey.
In Canberra, a couple of
elements that could contribute to HS decline come to mind. Most specifically a
decline in nest site availability. In my suburb of Ainslie (and replicated in
many other areas) there is hardly a house left that hasn?t had a renovation in
recent years, thus removing bird access to roof space and wall cavities for
nesting (the same would apply to Starlings). Older houses are pulled down
altogether to make way for townhouses and apartment blocks. When I did a survey
of breeding birds in Ainslie over 25 years ago, I noticed even then that some HS
were nesting in tree hollows, indicating plenty of food, but lack of prime real
estate.
Landscaping around houses has also changed
significantly over the years, most changes not favouring the HS: conversion of
gardens consisting just of lawn/rose bushes or privet hedges to native gardens,
reduction in lawn area, covering of ground with wood chips, mulching etc. These
landscape changes may often affect the composition of the bird community,
perhaps offering Sparrowhawks a more diverse range of prey thus favouring it while at the same
time placing the HS at a disadvantage.
No doubt, other factors play a role as well. Just
looking at the trend graphs for two species without knowing what else goes on
may easily result in erroneous conclusions. I well remember my counts of the
number of breeding Pied Currawongs versus the number of breeding Willie Wagtails
at the ANU. Currawong numbers went up over the years while those of Willie
Wagtails declined. It looked like a definite expected and linked trend, until I
realised that various landscaping measures (removal on many of the favoured nest
trees for Willie Wagtails over the years and taking a couple of years of
rejuvenating one of the ovals, a main feeding area for several pairs of Willie Wagtail during the breeding
season) could have contributed equally to their decline. Or, with the landscape
measures + the growing Currawong presence, there was little chance for a healthy
Willie Wagtail population.
Michael
Lenz
