RE: Red Wattle-birds

To: Geoffrey Dabb <>
Subject: RE: Red Wattle-birds
From: martin butterfield <>
Date: Sun, 16 Aug 2009 15:00:11 +1000

Following Geoffrey's suggestion I have attached not 1 but 2 graphs derived from the COG Garden Bird Survey of which I am the Coordinator.  I hope the list manager will not be too unhappy with me for slightly exceeding the 100Kb attachment size.

As you are a new member here is a bit of detail behind the graphs.  The graph most directly relevant to your question is the one titled 090849 RWB A Values.  RWB is my acronym for Red Wattlebird.   A stands for Abundance and is calculated as the number of birds reported in the Survey divided by the number of Observer weeks.  In this case I have both values for each week of the year (week 1 is the week beginning January 1) over the 27 years for which we have complete data.

What this graph shows is that there are 3 peaks in the number of RWB observations.  My interpretation (noting Geoffrey's caution from HANZAB) of the peaks is that:
  • the one in January (about week 3) marks the outcome of breeding in Canberra;
  • the major peak in April/May is flocks of birds passing through on their way out of the mountains; and
  • the second minor peak (September) is birds passing through on the way back to breeding territories (we are currently at week 33 building back up to that peak).
The second graph (RWB A and raw count) contrasts the value of A (LH axis with the total number of birds reported (RH axis) and shows that while the two series correlate extremely well (R=0.94) there is a small bias in the number of birds reported in July (as people may not pick up their GBS Charts until the August Meeting of COG) and a larger downward bias in that number in late December as many COG members, like most of Canberra, head off to the Coast.  These biases are smoothed out by the use of A.

I hope this helps rather than confuses.




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