As a long-time estimator of seabird numbers I'd like to make the point that
we don't believe that the daily totals are the actual numbers of birds
present in the vicinity of the boat during the day. They are the number
sighted and are a measure of relative abundance. The most important aspect
is to be as consistent as possible taking into account the arrival and
departure of groups etc. Statistically rigorous methods of recording
seabird abundance produce data that are not appropriate for general
broadcast. For example, a Sooty Albatross may pass close to the boat
giving a brilliant view but outside the sampling sector and or outside the
temporal sampling window and thus not appear in the data because of its
insufficient abundance.
Peristent boat followers are problematic. For example, Buller's Shearwater
will follow a boat for hours but will drfit away to a considerable distance
and then return enthusiastically as if it were a new arrival. I have seen
this happen up to 5 times so 1 sighting could have been recorded as 5
sightings, if the bird were to disappear from view. Reporting the
observations as 5 (5) confirms the presence of 5 individuals, 5 (2)
confirms the presence of at least 2 individuals, while 5 (1) conveys the
possibility that the number of individuals is anywhere between 1 and 5 for
the day.
Rohan's report clearly conveys the fact that SPPL were present in
significantly higher numbers than usual. SPPL generally aren't overly
interested in boats so I think Rohan's estimates are entirely reasonable.
SOSSA's at-sea capture/recapture programmes indicate that the 360° sighting
totals are generally a significant underestimate of the number of birds
present.
Dr P.J. Milburn
Biomolecular Resource Facility
Australian National University
GPO Box 334
Canberra
ACT 0200
+61 2 6125 4173 'Phone
+61 2 6125 4326 FAX
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