Thanks
both.
A
quick response:
The
measure Paul describes as "For the GBS, counting the number of weeks the species
was recorded each year, divided by the total number of weeks recorded that year,
to arrive at a percentage." This is the R% statistic, as given in every ABR
and explained and defined as such in The GBS Report. Assuming of course and I
hope that he means the number of observer weeks, not the number of weeks!
As the number of weeks is limited to 52 but the number of observer weeks is
about 1800 to 3500 per year. It is curious that the data graphs from
Paul start in 1990 and not 1981 but maybe just the earlier years don't have
non GBS data for the comparison.
Geoffrey comments that these "GBS reporting
rates" from Paul are similar on graph to the
graphs of the A statistic from The GBS Report. And correctly so, as these are
species that are almost always observed as singles or maybe pairs. By
definition, if every observation was of one bird (or even any random
number, provided it was always the same number), the graphs for R (Recording
Rate) & A (Abundance) would be identical in shape (different axis
labels if the number was higher). The closer G (Group size) is to one, the more
similar the graphs are. Note that G=1.19 for Golden Whistlers (GW) and
G=1.21 for Rufous Whistlers (RW), so they are similar and close to one. For
most species, the G stat is generally stable over the years). It is
abundance and social behaviour of birds that creates the difference. The
relationship of the GBS reporting
rate and the GBS abundance is discussed in
great detail in The GBS Report, esp. pages 31 to 34. That is the single most
important item from the whole GBS, in terms of understanding bird surveys.
Yes
the drop of GW R% following the fire year is dramatic and then following very
dry years. Tempting to make a link there. I wonder. There would have been much
habitat loss in the high country and likely lowered forest productivity. It
really would be useful to update The GBS Report to see what has happened since.
Remember that broadly speaking, for the GW for GBS years,
two years of presence are combined into one data point but for the RW each
single year exists as one data point. This is because the GBS year goes from
July to June and the GW is in Canberra more in winter yet the RW is in Canberra
more in summer. Again this is explained in The GBS Report, pages 29
& 30 & Figure 11 & Table 3.
Geoffrey writes about "a variable not brought out in that
table would be the number of sites reporting presence in the same
week". That may be interesting. I don't recall that question
ever being asked before about any species and I had not thought of that
one. The number of sites reporting presence in the same year is routinely
provided as the F% statistic and given rather clumsily and inconsistently in
recent ABR but this is something else. I suspect that would be a very difficult
query to design as to how to answer that
one.
For
what it is worth, this year (although there is still the second winter to come)
is I believe for the first time ever, that I have reached April with
no Golden OR Rufous Whistlers on my GBS chart.
Philip
Thank you for taking the trouble
to do this Paul. Very interesting. With no expertise, least of all
in mathematics, I am not deterred from offering the following
comments.
First, your graph on the GBS
reporting rate broadly mirrors Philip’s based on abundance – for the comparable
period. Philip goes back 10 years earlier, but stops at mid2002.
Your graph for the more recent years might well indicate a reduction in GW
numbers in gardens due to the fires (January 2003). I note from the ABRs
that GBS numbers for the GW were –
20012002: sites 69.5%, weeks
45
20022003: sites 72.4%, weeks
38
20032004: sites 73.3%, weeks 52
(postfires dispersal?)
20052006: sites 52.9%, weeks
43
20092010: sites 54.55%, weeks
37
Not all that dramatic, but a
variable not brought out in that table would be the number of sites reporting
presence in the same week, which should be reflected in your reporting rate –
and in the abundance figure, since most observations are of a single bird. So
one theory would be that since the fires (2003present) there has been a decline
in garden GW numbers that parallels the earlier decline (19872001) in
(for whatever reason) RW numbers. I would be interested in the 2011
figures – and in those for the current year which I feel might well show an
upswing if other gardens are like mine.
As to the other surveys, these
show, I would say, the RW going along at a 510% reporting rate and the GW at a
1520% rate – both pretty steadily and no doubt with their respective seasonal
(and habitat preference) variations. It does raise a question why the GW
should have declined in gardens but not in woodlands ….
From: Paul
Fennell [ Sent: Wednesday, 25 April 2012
12:09 PM To:
'canberrabirds chatline' Subject: [canberrabirds] Whistler Reporting
Rates
Hello everyone, including whistlers, tweeters, and
chirpers!
I have constructed two graphs, one from the GBS data, and one
from all nonGBS surveys, showing the reporting rates of both Golden and Rufous
Whistlers.
I calculated the reporting rates by:
For the GBS survey, counting the number of weeks the species
was recorded each year, divided by the total number of weeks recorded that year,
to arrive at a percentage.
For the other surveys, counting the number of data sheets
recording the species each year, divided by the total number of sheets recorded
that year, to arrive at a percentage.
The two graphs show some interesting ups and downs, including
one would suspect, the effect of the bushfires on Rufous Whistlers. I
leave it to you to make further interpretations.
Cheers
Paul
Paul Fennell
Editor Annual Bird Report
COG Databases Manager
026254 1804
0407105460
