used to measure the combined effect of ambient temperature, microclimate, and
wind on their rate of heat loss (Bakken et al. 1981; Wiersma and Piersma 1994).
11.3.2Rainfall and soil wetness
As with temperature, rainfall can affect birds directly by effects on thermoregulation
or flooding of nest sites or habitats or indirectly via effects on animal prey or the
growth and seeding of plants. Analysis of bird population size and demographic
parameters in relation to long-term rain-gauge records from networks of meteoro-
logical stations has yielded many insights into the factors affecting populations
(e.g. Peach et al. 1991). Records of water levels in seasonally flooded wetlands can be
used to estimate direct and indirect effects on bird populations and may usefully be
combined with measurements of flood extent from satellite imagery (Nott et al.
1998). The water content of soil affects birds indirectly by influencing the abun-
dance, activity, and depth distribution of soil invertebrates. The depth of the water
table below the soil surface can be measured by reading the water level in permanent
dipwells (pipes 6–50 mm diameter drilled with say 4 mm holes at least every 10 cm)
or temporary dipwells using a 2–5 cm soil auger and recording at 30-min intervals
until the level has stabilized. The water level can be measured using a ruler and torch
or an electronic dipmeter. Water content can be measured by weighing, drying, and
then reweighing samples of soil, but this is time consuming and rapid measurements
of soil wetness in relation to depth can now be made using a theta probe (Gaskin and
Miller 1996). Approximate estimates of soil moisture content can be obtained using
daily rainfall records and a water balance model and these have been found to repre-
sent the availability of earthworms to foraging birds reasonably well (Chamberlain
et al. 1999; Green et al. 2000).
11.3.3Slope, aspect, elevation, and topography
Slope, aspect, and topography affect birds via influences on the local climate,
including the exposure of an area to winds and can be quantified using contour
maps. Topography and wind direction also affect the availability of updrafts to
soaring birds. For direct measurements in the field a clinometer and compass can
be used to measure slope and aspect while elevation can be measured using a GPS
or altitude meter.
11.3.4Type, chemistry, and penetrability of soils
Soil characteristics can influence the distribution and abundance of birds by
influencing the effectiveness of their camouflage or that of their eggs and chicks
and by effects on vegetation or invertebrate prey. Detailed soil maps are available
for some regions and there are sometimes strong associations between bird
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