The trend described is a very old one and certainly is what others have found. Even given that, GBS data shows that the trend has reduced. This information
from The GBS Report follows: Note the monthly graph given on page 90, a luxury granted for only 3 species where the change in seasonal trend over time was worth highlighting.
Philip
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 were 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errors, are inconsistent in method and are superseded by this report.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.
From: casburnj [
Sent: Sunday, 26 July 2015 6:09 AM
To: canberrabirds
Subject: [canberrabirds] "When the Currawongs come Down From the Mountains"
This was of interest to me.
I heard a Slim Dusty Song this morning. Paraphrased:
When the currawongs come down from the mountains
It’s time to put the oil skin on
Its time to put the fire on etc
I notice on my Garden Bird Survey that in June and July were the largest numbers of Currawongs – 20 seen flying over my house in July. This is apart from the stayers.
Is this what others have found?
Jean
