[wisb] Re: Rarities
- From: Stuart Malcolm <stuart.malcolm26@xxxxxxxxx>
- To: Wisbird <wisbirdn@xxxxxxxxxxxxx>
- Date: Tue, 8 Nov 2011 16:56:39 +0000 (GMT)
Hi All,
I remember reading this article back in the UK and somebody posted this
commentary on an online forum:
"In the March '97 issue of British Birds there was a paper by Pete Fraser
called "How many rarities are we missing?". Fraser analysed the BBRC statistics
for 1958-92 and found that, despite steadily increasing observer coverage over
the years, there wasn't much variation in the percentage of rarities found at
weekends (44% in 1958, reducing to 40% in 1992). Sites well-watched by weekend
visitors from the big connurbations showed the most weekend bias; those covered
more exclusively by local observers the least.
He then devised a model that ran thus: Assuming that no site is covered with
100% daily efficiency, the rarities found on any given day will consist of (1)
a percentage of that day's arrivals, (2) a percentage of the previous day's
arrivals which were not discovered then and (3) a (small) number which arrived
even earlier, also previously undetected. At well-watched sites he reckoned
that 90% of all rarities were discovered, of which 40% were recorded only on
one day. At less well-watched sites 70% were recorded on one day only, probably
because with less coverage it is easier to lose track of the birds and because
fewer are found on the day they arrive. He ended up with an estimate that 60%
of rarities remain beyond one day; of these 70% are still present the next day,
70% of those the day after and so on.
Calculations were a question of first finding the best fit between the model
and the statistics. In the Isles of Scilly the reduction in numbers of birds
found on Monday pointed to an efficiency of 92% on Sundays and less effort on
Saturdays (76%) than on weekdays (78%) - which seems logical since Saturday is
the big change-over day there.
In 1990-94 the average number of rarities discovered each year was 755, while,
at a conservative estimate about 800 were missed. The rate of discovery for
passerines, near-passerines and waders was significantly less than for larger
birds (unsurprisingly). For passerines, the percentage missed in different
areas were:
Fair Isle 11%
Scilly/Cape Clear 11%
Orkney/Shetland 45%
South Coast 53%
East Coast 57%
Elsewhere 60%
Anyway, on Fraser's calculations (which I'm sure he wouldn't want taken too
seriously) it seems that we're finding less than 50% of the rarities."
This is obviously from the UK with a much larger number of observers in an area
the same size as Wisconsin (there will be tens of thousands of birdwatchers out
in the UK on a weekend, with maybe a few hundred in Wisconsin?). There are
probably more unusual birds to be found in the UK due to it's island position
and there is much less 'natural' habitat for the birds to hide in. From the few
months I've spent over here I would think that a very very small proportion of
the potential rarities in Wisconsin are found due to the low observer coverage.
As an example would the recent Kittiwake have been seen if it hadn't flown in
front of the Vermillion Flycatcher?
I would be inspired to go out and find some more if it wasn't such a horrible
day!
Stuart Malcolm
Appleton
________________________________
From: Janine Polk <j_l_polk@xxxxxxxxx>
To: wisbirdnet <wisbirdn@xxxxxxxxxxxxx>
Sent: Tuesday, 8 November 2011, 10:34
Subject: [wisb] Re: Rarities
There's no way to estimate numbers, but the rarities found by birders must
represent just a tiny fraction of what's actually there.
Janine Polk
Eau Claire
--- On Tue, 11/8/11, Al Schirmacher <alschirmacher@xxxxxxxx> wrote:
From: Al Schirmacher <alschirmacher@xxxxxxxx>
Subject: [wisb] Rarities
To: "MOU" <mou-net@xxxxxxxxxxxxx>
Cc: wisbirdn@xxxxxxxxxxxxx
Date: Tuesday, November 8, 2011, 6:36 AM
The recent rash of Midwestern rarities drives the question, what percentage of
rarities are seen by birders? Does the presence of a Vermillion Flycatcher
mean that ten are in the Midwest? How would one answer such a question?
Al Schirmacher####################
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