Pomeroy (1992) developed a rather more elaborate 1-h procedure in which
birds are listed according to which of the six 10-min divisions they are first recorded
in. He then weighted them (6,5,4,3,2,1) on the grounds that those detected in the
first 10 min will on average be commoner and called the resulting statistic a
Timed Species Count Score. In a later development (Freeman et al. 2003), the
distribution of timing of first detections was used to estimate encounter rates.
Encounter rates in this case mean number of detections per unit time rather than
number of individuals. Encounter rates can of course be measured directly, but
this involves more writing time in the field than the timed species count
approach because every detection has to be logged rather than just the first detec-
tion of a species. There is also the difficulty in recording encounter rates of having
to work out which detections are of a bird already recorded and exclude them. The
estimated encounter rate is probably superior to the timed species count in lack-
ing the arbitrary weighting coefficients and coming closer to being proportional
to relative abundance.
With listing methods, the end result for each species is expressed as the
proportion of lists in which it occurs. Commoner species will clearly be recorded
more frequently than rare ones but there is no reason why the relationship between
frequency and absolute (unknown) abundance should be linear. Indeed it almost
certainly would not be. No effort is made to deal with the fact that species vary in
how detectable they are, even when present. Thus the paucity of owls might not be
due to the fact that they are rare so much as the fact that they are hard to detect,
especially during daylight. In addition, very common species are likely to occur on
most or all lists (the more so the longer the recording time). As a result, the method
is not so good at separating the relative abundances of the most common species.
An obvious way round this is to shorten the recording time and collect more lists.
Encounter rates (total number of individuals divided by time) are rather better
than frequencies for separating the relative abundances of the more common
species which will tend to having frequencies close to one (they occur on most
lists). The disadvantage of encounter rates is that the field recording is greater.
There is a natural tendency to prefer to give time to finding new birds than
spending a lot of time writing in the field.
1.5 Standardizing the effort by McKinnon’s list method
McKinnon has proposed an alternative method of standardizing effort by repeated
accumulation of fixed length species lists (McKinnon and Phillips 1993). The
observer writes down each new species occurrence until a target number of
species has been recorded. At that point, a new list starts with all species again being
Standardizing the effort by McKinnon’s list method| 7