The information that Martin refers to, is available in my book: “Canberra
Birds: A Report on the first 21 years of the Garden Bird Survey”. This is a
detailed 130 page analysis of the GBS, that has been run by Canberra
Ornithologists Group (COG) in Canberra, continuously since July 1981. Martin
has referred to 30 years, which I can suggest is easily done by noting that
the next 9 (+) years have presumably continued the same trend as was therein
documented for the first 21 years, for those two and indeed many other
species. There is however no compiled analysis since the first 21 years.
What Martin wrote is correct. But it is unrealistic to describe the other
factors in play, in an email in isolation. The book, indeed the GBS
analysis, from its inception (1981), has gone to great lengths to adjust for
observer effort. As Martin has also had a long involvement with the GBS and
is well aware of the analysis (especially during those 9 + years) and has my
book, I am intrigued with his stated surprise that “adjusting for observer
effort doesn’t affect the pattern of numbers recorded greatly”. Also the
months mentioned should not be taken in isolation but as a continuum through
the year. When seen as a graph the sequence is seen as smooth as part of a
regular cycle. Among the 221 species recorded in the first 21 years (now
several more added since) there were more than 100 species with sufficient
data to describe an annual pattern. The text of the book lists the changes
in annual abundance patterns, for each species, inasmuch as data was
available. In each case comment on expected other factors at play are
offered. Thus there are about 100 different available stories, although many
species show similar annual changes. When seen in a fuller context it is
clearer.
Philip
—–Original Message—–
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