Suction traps often yield large quantities of insects, which would be difficult to
sort and identify, but the daily total volume of the catch can be a useful measure
(Bryant 1975). Insects flying near the ground can be sampled by flight intercep-
tion traps (Ausden 1996) or by counting insects seen through binoculars during
a watch of a standard volume of air for a fixed time (Flaspohler 1998). Visual
counts of the moths and dipteran flies active during the night that are the prey of
Nightjars Caprimulgus europaeushave be made using a vertically oriented spot-
lamp that is switched on briefly to minimize the attraction of insects to the light
(Bowden and Green 1994).
Sweep nets can provide a useful quick way of assessing relative invertebrate
abundance in dense vegetation. It is essential they are standardized, for example,
10 sweeps of constant strength and then the invertebrates counted.
The abundance of soil invertebrates can be assessed by hand sorting or otherwise
separating them from the soil cores. However, this may include animals that are
inactive or too deep in the soil to be available to the birds. Chemical extraction of
soil invertebrates by applying a solution of an irritant chemical such as mustard to
a quadrat in the field (Ausden 1996), appears to measure a combination of abund-
ance and activity or proximity to the surface because the number of earthworms
extracted per unit area shows short-term variations that are correlated with soil
moisture levels that affect earthworm behavior (Green et al. 2000).
The availability of plant foods such as leaves and seeds can be measured by
counting plants or their parts in quadrats or along transects. This approach is
appropriate for seed-eating bird species that mainly take seed while it is still on
the parent plant, but for species that take seed from the soil surface it is necessary
to scrape a layer of soil from a measured area and separate the seeds by sieving.
The seeds can then be counted and weighed. The contents of many seeds in the
soil seed bank may have rotted away or have been removed by soil invertebrates
and been replaced by soil. Hence, each seed (or a sample) should be crushed to
check that it contains endosperm.
For ground-layer plants that are the food of grazing birds, counts of leaves or
seedling cotyledons can be done in quadrats or along transects. It may be useful
also to score plants for signs of damage from grazing birds to give a measure of
utilization as well as availability. This is especially useful when the plant struc-
tures being eaten vary little in their number per plant, size, and shape so that the
researcher can easily judge what is missing. For example, by carefully scoring
damage to the paired cotyledons of seedlings being grazed by Skylarks Alauda
arvensis, it was possible to estimate the species composition of the diet and the
dry weight of cotyledon material of each species being eaten per day. The diet
species composition results agreed closely with an independent assessment based
264 |Habitat assessment