We could try a Capture / Recapture approach but that would take Simply Ages
and upset too many birdwatchers.
Let's do this statistically, making a whole bunch of scarcely warranted
assumptions, and grandly ignoring all sorts of scientific stuff, on the
assumption that what we gain on the swings we will lose on the roundabouts.
Although it's really difficult keeping your binoculars steady on a swing.
"Quality Birdwatching" (QB) is what we are interested in and that is done
with bins round your neck in the field, making you rather recognisable.
Real Birdwatchers (RBs) don't allow dust to gather on their bins. They use
them at least 1 hour per week; and allowing for annual 24 hour Twitchathons
etc, a RB spends 100 hours per year doing QB.
RBs spend about 50% of their QB time in recognised "good twitching areas"
(GTAs) and 50% across the road / down the street / round the corner /
wherever they happen to be. So each RB spends 50 hours per year at GTAs.
RBs always begin and end QB on the hour, spend zero time traveling between
GTAs, spend equal time at each GTA in their area, always manage to spot
other RBs present at their GTA of choice, and take no notice of M. Poisson.
Non-RBs are easily recognised by the dust on their bins, and a RB is very
observant, if not necessarily very sociable.
13.7 hours per day are viable for QB. Each hour of the day is equally likely
to be used for QB. That's 13.7 x 365 = 5,000 hours potential QB time per RB
per year.
So RBs spend 50 / 5000 = 1/100 of their available time in QBGTA and have a
1/100 chance of QBGTAing at the exact same time as another given RB and a
50/100 chance of doing so at some stage during a given year.
I live in Brisbane. There are 100 GTAs in or near Brisbane. I have a 1/(100
x 100) probability of meeting a particular RB during a given hour's QB or
50/(100 x 100) = 1/200 probability each year.
So the number of RBs in Brisbane is 200 x the number of RBs I meet at GTAs
per year. (Vagrant RBs from other areas cancel one another out.)
I've been here 3 months so far and have met 1 RB at a GTA so there must be
800 RBs in Brisbane.
There are 1,000,000 people living in Brisbane, and 20,000,000 in Australia.
RB and GTA density is uniform nationwide,so:
There are 16,000 birdwatchers in Australia.
And if you believe that, you'll believe I have Flamingoes nesting in my back
yard.
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