distribution and soil type (e.g. Green et al. 2000). Maps that combine information
on soils, topography, and climate in order to assess the suitability of land for
arable agriculture or forestry may also provide useful information. The density
of breeding Sparrowhawks Accipiter nisusin British woodland is positively asso-
ciated with an index of the suitability of the landscape for agriculture. The mech-
anism of this effect is that the small bird prey of Sparrowhawks is more abundant
on more productive land (Newton 1986).
Soil invertebrate abundance varies with soil characteristics, especially acidity,
which can be measured using a pH meter. Mix soil with twice the volume of
distilled water (pH 7) and wait for 10 min before taking the reading. Soil pH
depends partly on the type of soil, but it is not readily predictable from soil maps
because of the effects of agricultural management and the accumulation of leaf
litter. Earthworms tend to be less abundant in acid soil (low pH) and measure-
ments of both pH and earthworm abundance have been found to be good
predictors of habitat preferences of earthworm predators such as the Woodcock
Scolopax rusticola(Hirons and Johnson 1987). Some birds that feed on inverteb-
rates in soil or intertidal substrates do so by probing with the bill. For these
species the ease with which the substrate can be penetrated may influence the
suitability of habitats for foraging. For example, the penetrability of wet grass-
land soils, was a more important determinant of the duration of the breeding
season and foraging site selection in the Common Snipe Gallinago gallinagothan
prey abundance (Green 1988; Green et al. 1990). Penetrability can be quantified
using commercially available penetrometers. A much cruder approach is to drop
a graduated pointed stick from a constant height and record the depth to which
it penetrates.
11.3.5Water chemistry
The chemical composition of freshwaters influences birds mainly by affecting
the animals and plants on which they feed. The population density and breeding
success of Dippers Cinclus cinclusalong freshwater streams in Britain is strongly
correlated with water acidity because stream acidity affects the aquatic inverteb-
rates upon which the birds feed (Ormerod et al. 1991). The pH is measured with
a pH meter or, less accurately, indicator paper. In coastal lagoons the salinity of
water, and fluctuations in it over time, has strong effects on plants and animals
that may be important to birds. Salinity can be measured using a portable
conductivity meter.
Eutrophication of rivers, lakes, and shelf seas by discharges of water contami-
nated with nitrogen and phosphorus from agricultural fertilizers, sewage, and
wastes from livestock, also affects the food supply of birds by influencing the
256 |Habitat assessment