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