Hi Jack,
Well I don't know about your place but here is the extract from my book:
"Canberra Birds: A Report on the first 21 years of the Garden Bird Survey".
This is a 140 page detailed report on the population trends of all of our
birds. It includes in a way that no other book I know of, information about
bird abundances by month and long term trends. It is also a detailed
description of the survey and very useful to anyone planning or doing a bird
population study.
Page 81.
Pied Currawong Strepera graculina
This species is primarily a forest inhabitant but it has done very well in
the suburban and even urban environment. It has shown an interesting
pattern. Its monthly pattern has changed, yet when looking at the statistics
compiled on an annual basis, this fact is almost hidden. Allison (1993) used
the statistics then available from the COG ABR on the Pied Currawong and a
selection of small bird species, to explore whether there was any evidence
of increase in the Pied Currawong population or evidence of adverse impact
of this expected increase, on other species. The conclusion was negative in
both aspects but unfortunately this well-intentioned analysis missed the
point. The data he used was only for nine years, the ABR contain errors, are
inconsistent in method and are superseded by this report. Also he did not
have access to the GBS data now available nor fully understand what he had.
He did not appreciate that to answer the question posed, requires an
examination of changing abundance of this species over the summer period,
the breeding season, not the year as a whole. Even though overall the
numbers have stayed within the same range and the annual numbers still show
a smooth cyclic pattern, it is the steady increase in summer numbers that
has the greatest impact on other birds.
The Pied Currawong was historically characterised by very obvious peaks in
winter when they formed large flocks, with marked drop-offs for summer, when
most left the area to go into the high country to breed. GBS data averaged
over all years shows a low from October to February, rising smoothly to July
then declining smoothly to the November minimum. Over the years the
amplitude of the variation of this pattern has reduced enormously (see extra
graph). There is a possibility that the change in the chart instructions
from that on versions 1 & 2 that allowed for recording birds "outside your
area" to that on versions 3, 4 & 5 that did not, could have been partly
responsible for this. An examination of the changing average group size over
the 252 months of the survey shows that the size of winter flocks have
progressively become much smaller and the number of birds per observation
over the summer have increased. It is unclear why the winter abundance and
group size has reduced so much. Maybe the same number of birds are now more
dispersed through the suburban habitat. Perhaps the fact that the species is
now virtually a resident allows individual birds to maintain territories
which may reduce the size or dominance of migratory flocks. The birds'
behaviour has changed to maintaining social groups and breeding pairs
through the summer. As for abundance, the summer minimums have steadily and
smoothly increased to more than double over that period. Consequently only
in recent years the species has become a very common urban breeder, (from
17% of records in Year 1 to 60% of records in Year 18). This increase was
gradual. It is very likely that this will have a significant impact on the
breeding of other birds. The impact of this species on others is a subject
for concern or analysis, see Lenz (1990a), Allison (1993) and Bayly &
Blumstein (2001). Major et al. (1996) provided a clever demonstration of its
predation on nests, using artificial birds' nests. Much of the observed
predation is directed towards Common Starlings and Common Blackbirds.
In recent years there have been more breeding records by this species than
any other. Nest building commences in July, peaks in August and is finished
by November. Nests with eggs or young commence in August, peaks in October
and is finished by January, first dependent young in September peaking in
December and last in May. There are many records that appear to show a
complete breeding event and the duration is generally close to 15 weeks.
Graphs on pages: 90 and 103, Rank: 3, Breeding Rank: 2, Breeding graph on
page: 107, A = 4.02837, F = 98.70%, W = 52.0, R = 79.317%, G = 5.08.
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