Hi Steve,
That is a big change in the point being made, given the graph that was
shown. I assume then the graph shown includes the Atlas period. My idea was
based on supposing the species is recorded in 75% of suitable habitat forest
surveys and 5% of woodland, grassland & urban surveys. Then even if these
remain constant, given that I assume all results are pooled over time, and
knowing that the COG AOI has changed over the period, and assuming for the
sake of mathematics, that since the Atlas period the input rate of woodland,
grassland & urban surveys, compared to forest, has doubled, then that bias
alone could produce the change. There could still be a bit of that since
1992 but I have no knowledge about survey changes since then. So in addition
to Martin's suggestion to just give a graph for the period you are
suggesting, if you could refine that by including only the forest and
riverine habitat, that would tell a lot. If that is horribly difficult to
separate out then that is a pity.
Beyond that, taking the period from 1992 to the present, yes there looks
like a slow decline. This is in recording rate, which is not abundance.
Although as a dispersed species that holds territories and doesn't occur in
flocks, these phenomena would at least show a parallel. If there is a real
trend I would suggest likely to be related to being mostly fairly dry years,
as this is a species that prefers wet forest habitat.
Philip
-----Original Message-----From: wallaces Sent:
Saturday, 26 January 2013 12:02 AM To:
Subject: Re: Where have the Whiteface gone? Eastern Yellow
Robin
I should have been more specific. I was referring to the period from 1992 to
the present which excludes the atlas period.
Steve
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