2. Bird census and survey techniques
Richard D. Gregory, David W. Gibbons, and Paul F. Donald
2.1 Introduction
In Chapter 1, we saw how it was possible to use simple methods to assess the
species composition in an area and to give an idea of their relative abundances.
Here, we consider methods that will allow us to derive estimates of population
size or density or, where this is unnecessary or impossible, population indices.
Armed with such information over a number of years, we can then track changes
in population levels and, where appropriate, compare population levels between
different sites. As described in Chapter 1, the distinction between a censusand
asurveyis somewhat artificial, but here we use censusto describe a particular type
ofsurveythat counts the total numbers in an area (Figure 2.1).
2.1.1 What are bird surveys and why do we need them?
If we need a reliable estimate or index of the population size of a particular species
in a given area, then we must undertake a survey. There may be a number of
reasons for wishing to do this. It may simply be that, as the owner of a nature
reserve, we wish to know how many individuals of a particular species of bird are
present, or we may need baseline information for an area, or a species, that is
poorly known. If repeated at regular intervals, the counts allow us to track changes
in bird populations. Alternatively, it may be because a piece of land is being
developed (e.g. turned into an industrial area) and we need to undertake an assess-
ment of the likely impact of the development on the nature conservation value of
the land. Frequently, bird survey data are used to assess whether a piece of land
should receive legal protection from governments and their agencies; such desig-
nations are important to conservation because they are intended to constrain
potentially damaging activities. Information on population sizes of individual
species can also be used to set priorities, allowing conservation effort to be focused
on those species most in need of attention. In general, smaller population size is