individually marked breeding birds, it might be appropriate to pair each foraging
location with a site selected at random on the circumference of a circle centered
on the nest with radius equal to the distance of the foraging site from the nest.
11.6 Food abundance and availability
Birds feed on a wide variety of organisms and methods to measure the density of
all of them are beyond the scope of this chapter. Practical methods for estimating
density or an index of density for many taxonomic groups can be found in
Sutherland (1996). Rough measures of density may often be sufficient to answer
the required questions. We want to emphasize here that studies of the food of
birds require information not only on the density of food item, but also on their
availability to birds, which is affected by prey activity, protective attributes (such
as thorns, camouflage or poisonous compounds), depth in the substrate or
height above ground in vegetation. The researcher should find out enough about
the basic ecology of the bird to make choices about how to combine the measure-
ment of food abundance with measures of habitat that influence food availability.
One simple precaution is to ensure that the sampling of food abundance is being
done in the type of habitat in which the birds can forage. It is often the case that
birds have strong preferences for particular types of vegetation architecture in the
areas where they forage. Samples taken in places that have vegetation cover that
prevents birds from foraging their may give a misleading picture of food avail-
ability. Hence, pilot studies of the bird’s foraging behavior and diet are recom-
mended before large-scale sampling of food abundance and availability begins.
In some cases it may be appropriate to use a method that measures a combination
of abundance and activity. For example, the catch per trap per day of ground-living
invertebrates in pitfall traps is influenced by the activity of the animals as well as their
abundance. This would be a disadvantage in a study of invertebrate population
dynamics, but in a study of the availability of food for birds that locate prey when it
is active on the surface, pitfall trap catches could be a useful measure. However, the
researcher should be careful in deciding which invertebrates from those obtained in
pitfall to include as being available to the study’s focal bird species. A high propor-
tion of arthropods caught in pitfall traps are nocturnally active and thought should
be given to whether these are available to a diurnally foraging bird. This will depend
on the foraging behavior of the bird and the resting location by day of the prey.
The availability of flying insects to birds that catch them on the wing can be
assessed by powered suction nets with intakes placed high above the ground
( Woiwod and Harrington 1994). Catches from even a single trap have been
shown to be a good predictor of breeding parameters of local aerial feeding birds.
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