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Someone suggested that "The first words (rare, common)
refer to the likelihood of seeing a species in the ACT." I dispute
that the word "common" means (or should mean) "high frequency of being seen"
(this being another term for recording rate, except based on just the visual).
That is only a consequence. "Common" or not, should indicate population density
(or abundance). In contrast, recording rate is something
entirely different.
The first example that comes to mind is not a bird:
The program "Big Cat Diary" often makes the point that the leopard is by far the
most common of the 3 big cats in Africa, although it is much less often seen
than the other two, the lion and cheetah and that is because of their behaviour.
Back to local birds. The GBS Reports (pages 31 to 34) discuss this aspect in
detail. With graphs for 12 species comparing the monthly abundance from the GBS
to the monthly recording rate during the same 3 year period of the COG Atlas,
with accompanying text that explains why in some species the graphs look similar
and in others they are quite different. It relates to habitat use changes
and social behaviour changes over the year.
The GBS happens to highlight these things with
the A value, R value and number of records (which on one year is the F value)
statistics. It is wrong to think there is a direct correlation between the
abundance, observability and recording rate, except in the crudest sense.
My GBS Report presents a graph (Figure 33) of the relationship between
recording rate and abundance for all species on each year. I called it
"probably the most important result of the GBS". It is explained therein that
the relationship varies mainly according to the social behaviour of the species
concerned. Recording rate is strongly influenced by visual and sound
conspicuousness of the bird or its behaviour. Not to mention the enthusiasm
of people to submit records. Looking at the ABR, one would think many species
that we know to be common, appear to be rare, because people are not interested
to record them. But when we do get a Lewin's Rail at Commonwealth Park it may
appear on that scale to be almost common, as many people go to see it, so it
potentially has a high recording rate. Recording rate does not distinguish
between obvious species occurring at low densities and inconspicuous species
occurring at higher densities. I encounter the Wedge-tailed Eagle far more often
than the Yellow Thornbill but I am sure there will be more of the latter in the
ACT. Even though biomass, as in grams per hectare may favour the
eagle!
Philip
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