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Martin's graph is good and predictably similar to the breeding graphs
in The 21 Year GBS Report (page 106). Gives broadly the same impressions for the
2 species Martin has included (and is by comparison a cinch to produce). Martin's graph
gives more time detail by expressing by week, rather than month,
but less information overall as it only contains the DY stage (but that is OK
when that is the question, as here). However Martin's graph
is labelled as count of observations (so I accept that is what it
actually is). So the clear drop in week 52 (for both species) is surely due to
observers going away for christmas holidays, hence fewer observations. There are
other week biases, but week 52 is the most obvious. This week (14) will some
years include easter and that too will reduce the normal count of observations.
The data needs to be expressed relative to observer effort or it is deceptive.
The graphs in the GBS Report of course do take this into account to remove this
bias. To see the influence of the change in observer weeks, see Figure 6 of
The GBS Report (that clearly shows the impact of christmas and
easter). Martin's graph clearly shows Michael's observation to be
towards the end of the usual time range. That of itself does not mean that the
whole trend is different this year.
Figure
22 of The GBS Report examined the question about differences between years
in extent of breeding. The best that I could get from that data was a strong
suggestion that year 2 (a drought year) was low.
Both
Martin's graph and the GBS Report breeding graphs are clearly the results for
all years of the GBS combined. Neither purports to display this or any year
as different or the same as another year or an average of other years. I doubt
very much that the size of the GBS data set on any one year (like this year) is
sufficiently big to allow any statistical significance to breeding being later
this year than usual. It would be nice to be wrong on this. Having said that, it
is my impression also that it is.
Philip
Thank
you Martin, another informative graphic. When this year is analysed we
shall see the height of the W14 bar, but shall we ever be certain that a taller
bar represents an exceptional year. I would be interested to know whether
such a careful observer as Michael has any feeling that this year is
exceptional.
From: martin
butterfield [ Sent: Sunday, 10 April 2011
12:11 PM To: Michael and
Janette Lenz
Cc: chat line Subject: Re:
[canberrabirds] Late breeding
Last friday was in GBS week
14. See attached in support of these being late records.
Martin
On Sun, Apr 10, 2011 at 11:02 AM, Michael and Janette Lenz
<>
wrote:
On Friday,
8. April, at the ANU a pair of Red Wattlebirds with a fully grown but still
begging young. Parents fed it repeatedly.
On the same
day in Civic a pair of Galahs feeding a young that looked as if it had left
the nest hollow only very recently.
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