in the habitat concerned and allows species-by-species density estimation
(seeDetection probabilities). It is always preferable to record the exact distance to
birds, or failing this, distance within many belts, but in reality, this will often
prove to be impractical. As range-finders become increasingly affordable, they
open the way for simple and accurate distance estimation, especially for single
species surveys.
2.3.3 Line transects
At its simplest, a line transect involves traveling a predetermined route and
recording birds on either side of the observer. The distance a bird is seen or heard
from the transect line is normally recorded as an absolute measure, or in distance
bands. Distances should be estimated perpendicular to the transect line (rather
than the distance from the bird to the observer). Distance estimation of this kind
is key to the estimation of bird densities. Perpendicular distances can be estimated
in a number of ways:
- Distance is estimated by eye from the line, given practice and periodic
checking against known distances; fixed distances can also be marked unob-
trusively in the field using marker posts or colored tape to aid recording. - Observers may be able to visually mark the position of a bird when detected
and then use a tape or range finder to measure the distance when they are
perpendicular to where the bird was recorded. - Bird observations can be plotted on to high quality maps and the distance
measured subsequently. This requires good mapping skills and is helped by
having fixed markers in the field. - Observers can use a sighting compass to estimate the angle () between
the transect line and a line from the observer to the bird, and use a tape or
range finder to measure the distance (d) from that point to the bird. The
perpendicular distance is then calculated as dcos.
The sampling strategy chosen for a particular survey determines the sample
square or unit to be surveyed, but there is still the choice of line transect routes
within this area. There are several options, and some flexibility is advisable. For
example, a regular or systematic approach could be used with parallel transects
orientated north to south, or a series of transects oriented along the long axis
of the study area. A random approach, for example, with starting points and
directions of transects selected randomly, could be used. One could even use
a stratified random approach, for example, with the starting points and direction
of transects selected at random, but where each lies within an individual habitat
stratum. In reality, topography, watercourses, roads, certain land uses, and access
Field methods| 39