Sunday, 1 April 2012

Little Corellas and Sulphur-crested cockatoos

There has been some discussion today on the COG Chatline about the relative abundance in Canberra of Little Corellas (hereafter Corellas) and Sulphur-crested Cockatoos (hereafter cockies).   My first contribution was to present a graph showing the long term series of measures of abundance (A is the average number of birds seen per active GBS week).  Click on these charts to get a larger version.
Clearly the numbers of cockies are still, at this stage well above the numbers of Corellas.  Where the trends will go is highly speculative and I haven't pursued that further.  However one comment as a result of that Chart was to the effect that the Corellas may be coming in to the urban area as a result of the plentiful supplies of food in Autumn-Winter.

Thus I prepared another chart showing the % of birds of each species seen in each GBS week.  As there have only been large numbers of Corellas for the past 3 years what follows is restricted to that period.  The first graph is a straight presentation of the aggregate number of each species seen over those 3 years.
Unfortunately the result for Corellas looks rather like a profile of the country between Buchan and Kiandra and it is risky to make any strong statements about the comparison. However should I be asked about this by the Spanish Inquisition (or indeed any Inquisition) I would venture the guess that the Corellas seem to arrive in Late Summer-early Autumn and depart when the weather gets really cold. In contrast the cockies build up for a longer period with numbers reducing in early Spring.  If it is food that 'brings them in' one could hypothesise that
  1. the Corellas prefer a food that is available earlier in the season and they leave when that is exhausted; or
  2. as the cold weather becomes more intense the larger cockies (around 900gm rather than around 600gm for the Corellas) out-compete the Corellas, driving them away.
Since the discussion was initiated by an observation of a very large flock I wondered if the saw-blade appearance of the second chart was due to a few observations of very large flocks was skewing the charts.  I don't believe isolated large flocks of Corellas skew the data significantly since 11 of the 17 observations of >100 birds occur between weeks 16 and 22 (and thus such flocks could be considered 'normal at that time).  

This did lead me to look at the average group sizes reported through the year.  In the Chart to follow the original data is plotted as points while the lines are polynomial trends used to remove the noise from the original series.
Although the fit of the data for Corellas to the line is rather 'ordinary' it does suggest they build up in numbers rather more quickly than the cockies and peak a little sooner, before both species follow a similar reduction in group size by Spring.  I will leave it to the professionals to suggest the biology possibly underlying this.

1 comment:

Flabmeister said...

The points below were made to the COG chatline by Michael Lenz and are reproduced here with his agreement. I have slightly edited them to clarify references to other discussions.

{The} morning flock of Little Corellas may have come from the ANU roost where actually both species roost together (I think Leo Berzin provided to the chat line some figures on totals for both species from a mid winter count, last year). Of course, we do not know the size of the 'catchment area' for this population coming to roost at the ANU.

To judge by the L. Corellas I see crossing Lyneham in the morning (often in various directions), they may well travel some distance to their day-feeding grounds.

Regarding competition between the 2 species, due to body size they have different cavity requirements. The smaller L. Corella may have better chance of finding a suitable hollow (or may find a hollow in younger trees) than an SCrCockatoo may be able. The L. Corella may then be more likely to be in competition with the Galah, as was already mentioned in another message.