Some thought needs to be given to surveying birds that are non-territorial, semi-
colonial species, those that sing for brief periods, and those that have unusual
mating systems; but this is less of a concern than in territory mapping. A poten-
tial disadvantage of both transect methods for some purposes is that they tend to
follow paths, tracks, or roads and so may not be representative of the area as
a whole. A practical way around this using point counts is to establish counting
stations at right angles to the transect, and say 30 or 50 m into the habitat.
2.3.7 Detection probabilities
Having conducted a survey of a species in a particular habitat, it makes sense to
compare the results with those of other similar studies in order to place your
findings in context. This is often easier said than done, however, because to do so
using the raw, or “unadjusted counts,” you must assume that the probability of
detecting birds is the same for each data set that is compared. It is an inescapable
fact that some birds present in your study area will go undetected regardless of
the survey method and how well the survey is carried out. Detectability is a key
42 |Bird census and survey techniques
Table 2.1A comparison of line and point transects
Line transects Point transects
Suit extensive, open, and uniform habitats Suit dense habitats such as forest and scrub
Suit mobile, large or conspicuous species, Suit cryptic, shy, and skulking species
and those that easily flush
Suit populations at lower density and more Suits populations at higher density and
species poor more species rich
Cover the ground quickly and efficiently Time is lostmoving between points, but
recording many birds counts give time to spot and identify
shy birds
Double counting of birds is a minor issue, Double counting of birds is a concern
as the observer is continually on the move within the count period—especially for
longer counts
Birds are less likely to be attracted to the Birds may be attracted to the presence of
observer observers at counting stations
Suited to situations where access is good Suited to situations where access is
restricted
Can be used for bird–habitat studies Better suited to bird–habitat studies
Errors in distance estimation have a smaller Errors in distance estimation can have
influence on density estimates (because a larger influence on density estimates
the area sampled increases linearly from (because the area sampled increases
the transect line) geometrically from the transect point)