Pied Currawongs in Canberra and surrounds

There has recently been a discussion about the Pied Currawong in Canberra on the Canberra birds chatline.  At the time of composing this post the most recent article is an objective and helpful comment about the natural order of things. I will attempt in this post to show a few interesting observations about the species derived from the Garden Bird Survey (GBS) which I coordinate on behalf of the Canberra Ornithologists Group. 

A first point to illustrate is that the annual values of Abundance (denoted 'A' and defined as the average number of the species seen per site-week) are very variable.  Applying a 4th order trend gives a measure of correlation of 0.21 - showing the trend is not significantly different to 0.

A key pointer to what has changed is the distribution of the species through the year.  In the earlier years of the GBS (which I take to be the 10 years starting on 3 July 1981) Currawongs were present in Canberra all year but in lower numbers in the warmer half of the year.  (I use the term warmer half rather than Summer, because as shown below as far as the Currawongs were concerned there were effectively 2 seasons rather than 4.)  An equivalent distribution for years 20+ (multiplied by 1.1r to be equivalent to a 10 year aggregate) is also shown.

There is clearly a very different pattern between the two periods.  The next question I addressed was whether this was a sudden change (eg an effect of bushfires) or a gradual 'evolution' over the period.  From eyeballing the curve for the first 10 years above it appeared that the period where the numbers was higher in earlier years was week 12 (beginning 19 March) to week 40 (beginning 2 October).  I calculated the value of A (average number of birds per site week) for that period in each year and for the rest of the year.  A graph of the ratio of these two values of A shows a very strong relationship to time over the 28 years of the GBS (R2 = 0.96).

It is very interesting that the ratio has almost levelled off in the most recent period.  Noting that the value of A has also been more stable over the period I could hypothesise that we are approaching an equilibrium.

A feature that I have noted in the past is that the huge flocks of birds reported in the early years of the GBS almost vanished until the most recent years.  The number of reports of two large flock sizes are illustrated below.(The apex of each stacked column is obviously the number of flocks of 50+ birds.)

To some extent the "loss" of the large cool season flocks underpins a decline in Group size (G) evident in the GBS summary statistics.

Again the value of the statistic appears more stable in recent years. I have been unable to establish a reason for the somewhat precipitous drop in the value of G from year 11 to year15.  It has been hypothesised that this period spans year 13 in which year the GBS changed protocol to exclude flocks seen outside the GBS site.  I do not support that view:

• there are 90 charts which report for at least 1 of those 5 years;
• of those 30 Charts report for every year - 13 report a higher average number per year in years 13+ than in years 11 or 12 and 17 report a somewhat - but not hugely - lower number;
• of the other 60 charts reporting in at least 1 of these years there is a wide range of reporting patterns, but certainly no consistent reduction in reporting after Year 13 (and no single obvious outlier).

Overall these graphss show a few aspects of the statistics available for Pied Currawongs.  I will not try to interpret them in terms of their biological significance nor attempt to project them forward. 


Returning to the question "What to do about Pied Currawongs?" I am mindful of the comment in 'The Hitchhikers Guide to the Galaxy' that the answer to every question is 42.  However in the case of Pied Currawongs it is 47.  Please note that I make this comment not because of their unfortunate dining habits (which as Denis Wilson has pointed out is just Currawongs being Currawongs) but because the pair that nest in a tree in our lawn swoop the daylights out of me every time I walk out!

I will also say that I see there as being significant differences between Pied Currawongs, and Common Mynas.  The former is a species native to the Southern Tablelands area who have been smart enough to take advantage of the new environment offered by the genus Homo (every time I open a newspaper I question the specific addition of sapiens).  The latter were introduced here by the deliberate and extremely unwise actions of an individual (while we are thinking of  Homo ignoramus).  Had this not been done it is possible that the rats with wings would still be finding their way through the take-away wrappers beside the Federal Highway!