10. Diet and foraging behavior
William J. Sutherland
10.1 Introduction
Studies of diet and foraging behavior can help answer a wide range of questions.
What does a species eat? Why does it prefer certain areas for feeding? Is there com-
petition for food among particular species or among age classes or sexes within
a species? How many individuals can a site sustain? Is the food depleted during a
season? What are the consequences of habitat change? Answering some of these
questions also requires a parallel study of prey abundance (see Chapter 11).
Many of the methods described here involve storing food materials in alcohol.
Seventy percent ethyl or propyl alcohol is usually used but anything in the range
60–80% is suitable. Alcohol extracts water from tissues so either the material
should be a small fraction of the total volume or the alcohol should be replen-
ished after a day and again after a few days. Alcohol is highly flammable and
needs to be stored in fireproof containers. Thus, take only small quantities in the
field in leakproof unbreakable containers (not glass) or take the fresh samples
back to the lab. Alcohol also dissolves fats, so should not be used for preserving
material to be used to obtain dry masses or fat contents.
The diet varies through the day, through the year, and between years, sites,
sexes, or age groups and even between individuals. The sampling needs to be
designed to reflect this variability.
10.2 Diet composition
10.2.1 Direct observation
Useful information can often be obtained by directly observing where birds feed,
what techniques they use, and what their captured prey looks like. Such informa-
tion is especially useful when combined with an examination of the range of