11. Habitat assessment
William J. Sutherland and Rhys E. Green
11.1 Introduction
Most field studies of birds incorporate measures of habitat extent and quality. By
definition, an ecological study seeks to investigate trophic and other relationships
among different species and the relationships of species with abiotic aspects of the
environment. Autecological and behavioral studies of a focal bird species attempt to
identify the environmental factors that influence population processes and behavior.
Applied research directed at bird conservation usually attempts to improve under-
standing of habitat preferences and the relationships between demographic rates or
population density and habitat area and quality. Habitat is usually assessed either to
determine habitat associations or to document changes over time.
11.1.1Habitat associations
Habitat associations relate bird distribution data (e.g. presence, abundance, or
nest site location) to habitat data. One of the main methods, area comparisons, is
to select a range of areas and relate abundance or presence to habitat. An example
would be to select a number of blocks of mangrove and quantify both the habitat
and the number of birds in each. Abundance or presence could then be related to
habitat. Area comparisons are more likely to reveal the habitat associations if a
wide diversity of sites are used. This approach can be carried out on a range of
scales. At a patch scale the frequency with which different foraging patches are
used could be quantified and related to habitat, while at a site scale the density of
birds in different forest blocks or on different lakes could be related to habitat.
In other cases it is impractical to select sites beforehand and measure the habitat
in them all, either because the area is not readily divisible, or because the species
only occurs in few of the selected areas. The other main method, presence–absence
comparisonscomprises comparing areas used with either a selection of areas
available or a selection of unused areas. If there are a large number of potential areas