Following Andrew's reference to reporting rates in the Atlas I have had a
look at the abundance measures (mean number of birds per week per site) for
the Gang-gang cockatoo as reported in the COG Garden Bird Survey (GBS).
Starting in 1981 these show a low point in 1988 following a marked drop in
abundance from 1987. From then there has been overall growth in abundance
(although it displays a number of peaks and troughs so that applying various
trendlines to the data gives values of reliability that are not exciting).
The abundance 'jumped' in February 2003 (after the Brindabellas were more or
less completely burnt out) and has stayed relatively high since then until
June 2004, which could be taken as suggesting that the birds moved to lower
elevations after their preferred habitat was lost. However that doesn't
explain some of the other peaks and troughs.
Another factor that could be in play is changes in the location of reporting
sites or observers but I haven't assessed these. Also, the area covered by
the GBS only amounts to a small part of the range of the Gang-gang.
I wonder however if measures of abundance or reporting rate such as those in
the GBS or the Atlas are really the best thing for assessing a bird's need
for protection? Breeding success would be more important but the nest
related aspects of that appear not to occur in the GBS area.
Martin Butterfield
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