I will comment on this: I
believe Martin invented the idea of the polynomial (a
fantasy of a smoothed curve) that I assume tries to dumb
down the raw data when the trend is not a straight line over
time. I am not satisfied that this polynomial smoothing
process is useful as it assumes that variation between years
is actually less than what the results show. There is
absolutely no reason to believe or even contrive this to be
true. It is equally likely that the true variation between
years is actually more than what the results show. If
considering migrants, this is almost certain that this is
the case. Ultimately though there is no way to be sure. Also
I fail to see how that constructed curve shows anything more
or adds anything useful than just looking at the actual data
points on the graph. I agree with the suggested Extending
Martin's graph with the published 2011-2012 figures
indicates that the observable population, rather than
continuing to decline to zero as implied by the dotted
curve. It would be wrong to assume any particular
trend will continue. I made this very clear in The GBS
Report. As in a trend should not be extended into the future
without very good reason. Although I do not see evidence
and I hope Martin (or anyone else) was not trying to suggest
it would. Arguably the regression lines I put in the long
term graphs for some species in the GBS report were equally
limited value.
In response to other points. It
depends a lot on how mobile the birds are. The geography of
GBS sites does not restrict to home range of Mynas. They
will move even on a daily basis far more than the GBS site
size, so will be available for counting beyond the range of
nest sites. Home range of Mynas may vary over the year from
breeding times to non breeding times. It is very possible
that the particular conditions around the ANU campus
(especially number of nest sites) may be different from the
broader GBS habitat. Then again I believe that data that
Daryl has presented is based on yet another set of
geographical parameters (he says trapping at active
myna nest sites).
He says Interestingly,
annual recruitment, as reflected in the January trapping
records, follows a similar trend to the index values in
Martin's graph. I do not see any similarity in
these two graphs. The one given by Darryl is by month from
2006. "Martin's" is annually from 1981. It would be far
better compared to my monthly graph and text in The GBS
Report which does show a strongly consistent trend. That is
simply due to the annual breeding cycle. The graph from
Martin clearly shows long term population trends pooled by
year, nothing more or less. It shows nothing about the
annual breeding cycle and so is not similar and I don't
think is designed to do so.
Lastly credit where it is due.
"Martin's
population index" is not a fair
description. Martin compiled that graph (really the database
easily outputs it). They are GBS results. The GBS was
started by other people over 30 years ago and collected by
hundreds of people. The A value was a construct of Ian
McComas Taylor. The idea and work to create a database that
connects results over the years into one source was mine.
Until then results were only available on a single year
basis.
Philip
.
Martin's population index is based on observation across all
surveyed habitats - many of which contain few or no viable nest
cavities.
For comparison, the graph below provides a population index based
on trapping at active myna nest sites. Interestingly, annual
recruitment, as reflected in the January trapping records, follows
a similar trend to the index values in Martin's graph. Of course,
the majority of the new recruits are surplus to the breeding
population, which is limited by the availability of viable nest
sites. Any trends in the size of the breeding population would be
reflected by records collected in November when only breeding
adults are present at nest sites. This index has not changed much
since 2007.
Extending Martin's graph with the published 2011-2012 figures
indicates that the observable population, rather than continuing
to decline to zero as implied by the dotted curve, may have
started to plateau at a level between index values 1.0 and 2.0.
Given the increasing wariness of the population, it is likely to
remain at something like that level provided that the number of
viable nest cavities is not reduced further, or does not increase.
On 26/06/2013 1:38 PM, Geoffrey Dabb
wrote:
This
was the graph that Martin (I think) circulated last (?)
year
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