This
seems to me to be confusing the value of Steve’s kind of graph with the deeper
question: ‘What do COG’s records really show?’ - ie what limitations
should be taken into account in their interpretation. Of course the data
compilation is largely unstructured and subject to all kinds of biases.
Surely we take those into account in making use of them.
On
the face of it, there is strong evidence the Little Bittern is present here
mainly in the warmer months - although the recent sightings show that is
not an invariable rule. What is the true underlying pattern of
movement? - as for so many Australian species we do not know.
For many species, much of the reporting cited in HANZAB under ‘Movements’ points
in different directions.
Now
Philip questions the value of counting records month by month - as
in the graph. The graph is simply a quick way of showing what the records
tell us about reports over a year. These quick snapshots by Martin and now Steve
have added some interest to this chatline, and I for one appreciate them.
If (subject to Steve’s patience and goodwill) you wish to go deeper you can
investigate individual reports and their exact time and location -
gain an idea of their reliability, even. This will show whether a cluster
of reports in November are attributable to a particular year and to one or a
series of reporters, and – with a high degree of probability - whether
they relate to a single appearance, possibly whether more than a single bird was
involved.
In
short, the graph does not represent all the data held by COG. It merely
offers one way of looking at it. The question ‘How real are these data in
terms of the species presence?’ is not one that should be directed to the graph.
I
am aware Philip knows all this, and merely lacked his usual clarity in
_expression_.
From: Philip
Veerman [
Sent: Thursday, 12 June 2014
9:50 PM
To:
Subject: RE:
[canberrabirds] Little Bittern
Maybe
Mark is commenting on the overall level of records of the species in our AOI.
That is fine. I vaguely recall his story about When COG was
surveying for the then proposed site for the National Museum at Yarramundi
Reach, which I assisted in, ......
as I was involved in that too. I was commenting on the issue about whether the
implication to be taken from Duncan's graph is real (as distinct from
statistically correct) in suggesting there are more Little Bitterns here in the
warmer months. The aspect of being cryptic is very relevant in the situation of
a species being present in similar numbers all year but by its behaviour is well
hidden for part of the year and easily observed at other times. I don't know if
that is relevant for this species. But it could be and thus is a point I
overlooked. It could also be that people are more willing to proceed to wade
chest deep through the reeds during
warm weather than cold, and that could influence recording rates too.
As
Martin says, chatline mentions are not official records but they are a quick way
of establishing a pattern of dates. Also it is not needed for each report
(especially of repeats) to make it to the
Rarities Panel to be
officially on the record. Although them having several is better than none or
one if helps or it is important enough.
-----Original
Message-----
From: Mark Clayton
Sent: Thursday, 12 June 2014 2:44 PM
To: 'Philip Veerman';
'Wallaces';
Subject:
RE: [canberrabirds] Little Bittern
One
of the big problems with cryptic species is that they are just that – cryptic!
When COG was surveying for the then proposed site for the National Museum at
Yarramundi Reach, which I assisted in, I asked if anyone bothered to check the
reed beds for Little (now Black-backed) Bitterns. When told “NO” I proceeded to
wade chest deep through the reeds and successfully flushed a couple. Since then
there have been quite a few records, often by people using canoes to scan the
lake side edges of the reeds. Looking at Lake Ginninderra and the creek that
flows into it (past the Crace and Giralang ponds and close to the large McKellar
pond where the two current species are being recorded), there are large expanses
of reeds that could easily hide both species of bittern and that theoretically
are inaccessible except y boat.
I
don’t know about the south side of town but in the newer Gungahlin suburbs there
are numerous small reed and rush-lined ponds that would be quite capable of
hosting both bittern species. How many people visit these on a regular basis? I
don’t always agree with Philip but I think he is right in this case, with most
of the records being of the same birds being constantly reported by people
seeing them for the first time and putting them on the chat line. How many make
it to the Rarities Panel I don’t know but all records of these species should go
through the panel to make sure they are officially recorded as well as putting
in something like an “incidental report form” which can be found on the COG
website.
Mark
From: Philip
Veerman
Sent: Thursday, 12 June 2014 2:06 PM
To: 'Wallaces';
Subject:
RE: [canberrabirds] Little Bittern
Thanks
for that. So mainly a warm month visitor. (The first RAOU Atlas suggests that
too, the second RAOU Atlas is hard to perceive any trend). So is the current
winter record record unusual? It does beg the question though about counting
the
number of records by month. Are
these separate records? How real are these data in terms of the species
presence? In this species I perceive that there would be very few separate
events. The species is rarely here and when here many people go to find it. So
there may well be a lot of recounting of the same birds in this graph. Which can
make a trend look bigger than it should.
-----Original
Message----From: Wallaces Sent:
Thursday, 12 June 2014 12:48 PM
To:
'canberrabirds'
Subject: RE:
[canberrabirds] Little Bittern
Previous
records of Australian Little Bittern are all from October to March. The graph
below shows the number of records by month for the period 1 July 1981 to 30 June
2013.
Steve
