I looked into this further last night. Commendably with some of the
2nd atlas surveys, concurrently some surveys were done by the 1st
atlas method allowing comparison. Gang-gangs were one of the species
showing the largest difference between methods. The reporting rate for
Gang-gang was 40% lower with the 2nd atlas method (surveying the same
palce at the same time). A confidence interval isn't given but given
the accompanying p-value the CI must be large. So 40% should be taken
as very imprecise estimate.
Now Gang-gangs got their vulnerable listing because:
"Data from the Atlas of Australian Birds clearly indicate that the
Gang-gang Cockatoo has declined dramatically within NSW. A comparison
of the first and second Atlas of Australian Birds (Barrett and
Silcocks 2002) showed that between atlas periods (1977-1981 and
1998-2001), the overall reporting rate for Gang-gang Cockatoos
declined by 44% across its NSW range. .... Reporting rate
is considered an accurate index of distribution for such a readily
identifiable bird as the Gang-gang Cockatoo."
The last sentence is very wrong. Precisely because Gang-gangs are so
conspicuous reporting rates across atlases are not an accurate index,
and the new atlas indicates this clearly.
Unless I'm missing something the first sentence is also quite wrong.
The data is consistent with no change in Gang-gang abundance and
comparison between atlas doesn't shed much light or what might or might
not be happening to Gang-gang numbers - their NSW range looks stable
between atlases.
Andrew
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