Dusky Woodswallows & Noisy Friarbird

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Subject: Dusky Woodswallows & Noisy Friarbird
From: "Philip Veerman" <>
Date: Sun, 7 Sep 2008 17:56:37 +1000
Clearly, at this time of year the Noisy Friarbirds are arriving in Canberra and increasing in numbers towards their summer peak. The Dusky Woodswallows are also about to arrive here but they mostly pass through, and their peak as far as GBS results go, is in Autumn, after the breeding season. Note that both species have occasional records through the winter in spite of the general low numbers then. Martin's graphs are a GBS innovation, valid but don't confuse it with other methods of description. I found mention of "the F% value" confusing. I would point out that the GBS statistic F% has always been given as based a simple yes/no result on annual results (whether a species was recorded at least once at a site on a year and that calculated over the total of all sites for the year). As such it is a measure of distribution, rather than seasonal trends and F% doesn't fit into an analysis of within-year variation. I think Martin has subdivided the idea into 52 weeks and done the calculation of presence for each week. Taking the data as he has is fine but note that, for example the dip at week 52 in # of observations, for the Dusky Woodswallow graph is due to the presence of christmas to new year and hence fewer observer weeks in week 52, rather than anything to do with fewer birds.
As the GBS collects data on numbers of birds, which is what makes the GBS so special, I still prefer graphs of species trends to be based on the count data, (the A statistic) rather than things based on recording rate or proportion of weeks during which a species is observed (however derived).
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