The method that Martin has used “charted the total number of birds reported”.
That does not allow for that the sample size of observer weeks (which should be the denominator in any such calculation, is variable (although Martin has chosen to bypass that
and it may well be sort of excusably right that that using A values would give a similar pattern). Earlier COG ABR and The
GBS report based everything on data relative to the appropriate denominator. Actually the A values are given in the COG ABR but only in a way that it is very difficult to find, because they are not given in the text! As was the case years ago! These data can
only be found by scrolling through that long summary table, with no way of indexing it! How dumb…. Too tedious even for the most dedicated. I have had this discussion before.
However note that there is a huge component in the data Martin has used, of repeat observation of the same birds at the same site on subsequent weeks. In early
years the species did not stay around much so there was not much repeat observation of the same birds at the same site. Now they are becoming more resident, so a large part of the increase shown in the graph of data that Martin has used relates that change.
As in nowhere near most of the increase suggested is more birds, I suggest although sadly I don’t have the figures, it is mostly or overwhelmingly due more of repeat weekly counts of the same individuals, due to increased residency. As I said The GBS Report
needs to be redone so these things can be properly explained in context and alternate interpretations needs to be done in a manner that comprehends the data.
The suggestion of role of fire is interesting.
Philip
From: Don Fletcher [
Sent: Friday, 30 August, 2019 5:03 PM
To: 'Martin Butterfield'; 'COG List'
Subject: RE: [canberrabirds] Grey Butcherbird
Hi Martin
Your hypothesis appears to be that GBBs moved from burnt areas into suburbs, then abundant artificial food kept them there, including after the burnt areas recovered, thus creating an abundant urban population where only a small population
existed before. I don’t have any problem with that hypothesis (except it probably overemphasises
artificial feeding) but I’m wondering whether this chart evaluates it. Any ecological data set is subject to a large number of sources of variation. To my eye, the variation from the projected trend looks slight, as ecological data go, and that is referring
to data from well-designed, systematic counts in an experimental setting. And these are not.
As you have recognised implicitly, if GBBs were increasing before the fire due to some ecological process, we would expect the number in the
population to appear to upswing very sharply at about this time anyway, because population increase is exponential by nature. And the increase process, both pre-fire and post-fire, may have been more than demographic. Both stages probably included immigration
(birds moving to the city) as well as demographic increase (breeding in the city). In that light, the evidence that increase after year 24 was ‘more than simple demographics’ is less surprising.
An increasing population may generate even bigger counts because the counts are subject to operator effects, as has previously been acknowledged.
Furthermore, as I understand it, in the COG general database, the counts of many, if not most, species increase at about the same time as this (the uptake of ebird has been suggested as one reason but there are other possibilities as well,
including the effect of the 2003 fire on the people doing the counts (economic, social, time availability, attitude to life, etc) and the effect of the fire on the counting environment e.g. changed visibility due to more open vegetation, and later, thicker
vegetation. I cant quite remember but I thought that that general increase was said to apply to the GBS too. If it does, the general increase needs to be accounted for before we can truly see how much GBBs have increased.
Food for thought
cheers
Don Fletcher
0428 48 9990
From: Martin Butterfield <>
Sent: Friday, 30 August 2019 3:45 PM
To: COG List <>
Subject: Fwd: [canberrabirds] Grey Butcherbird
See below: I meant to reply to all but by accident only sent to Geoffrey originally.
---------- Forwarded message ---------
From: Martin Butterfield <>
Date: Fri, 30 Aug 2019 at 10:33
Subject: Grey Butcherbird
To: Geoffrey Dabb <>
I have looked at the first 30 years of the GBS data on Grey Butcherbirds and charted the total number of birds reported. (I suspect that using A values would give a similar pattern so haven't done
the extra calculations.)
The major growth appears in
year 24, after the 2003 fires leading to a view that they may have driven the Butcherbirds out of the Brindabellas. To sort-of test that I then charted up to year 24 and projected the trend forward 6 years. That gives a projected
number of Grey Butcherbirds in year 30 of about 120, rather than the 468 actually reported. I think this discrepancy is more than could be explained by simple demographics so it tends to support the idea of an infusion of birds from the fireground. Having
got to the urban area they found it to their liking, as well meaning retirees flick mince for them!
On Fri, 30 Aug 2019 at 08:43, Geoffrey Dabb <> wrote:
For those who came in more recently, the background is that this is a bird with a history. (Although you might reasonably think all birds have a history.) The 1992 COG atlas said
‘Despite intensive observation, there are only a handful of urban records. Their absence from the city is surprising in view of their ubiquity in larger cities such as Sydney and Melbourne; their low numbers in Canberra have been linked to the city’s poor
lizard fauna …’
Anecdotal reporters of the species usually have that background in mind. There is no doubt GBb numbers are increasing in the suburbs, for whatever reason. One is tempted to point
to feeding by suburbanites, although the appearance of the species near housing seems to be preceded by a build-up in numbers in adjacent woodland, according to my observations.
A pair is showing signs of nesting in the small neighbourhood park of Rocky Knob. They are of a confiding nature, coming to the backdoor for a scrap of flung meat. Below snap
taken just now of the pair in the Feijoa tree from 2m away. The upper out of focus bird has the dusky plumage of a younger bird. Their catching ability while on the wing is remarkable. A piece of flung mince from 2m is taken at the mid-point (ie 1m from
the flinger)and the bird is able to reverse direction with a couple of wingbeats before flying quickly away..
