found divided by the number eaten. Similar experiments can determine if
digestibility varies with prey size.
10.2.5 Pellet analysis
Many species regurgitate pellets comprising bones and other hard parts along
with fur and feathers. These provide an excellent means of quantifying the diets
of owls but are less useful for diurnal raptors which digest bone. Many other bird
species also produce pellets, particularly skuas (jaegers), gulls and waders (shore-
birds), and shrikes. Pellets differ from fresh mammal feces in that they neither
smell nor compress readily. Dried feces may look similar to pellets but the con-
tents of feces are more digested and they are normally longer with straighter
edges. Pellets may be found at roosts and nesting locations, as well as on the
feeding areas. Pellets can be stored by drying or freezing.
For analysis, the pellets are placed in water until they are easy to tease apart and
assessment is made the same way as for droppings (see 10.2.4). For larger prey the
head may not be eaten and it is then necessary to identify other body parts. The
problem of differential digestions of different species (see 10.2.4) can be even
greater for pellets than droppings. For example, Green and Tyler (1989) showed
that Stone Curlew pellets contained small mammal bones and hard parts of large
insects, but that remains of small arthropods and earthworms occurred only
in trace amounts, though remains of all prey were abundantly detectable in
droppings. Calibration trials with captive birds (10.2.4) is a solution.
10.2.6 Stomach analysis
Birds may be found dead and the contents of the stomach analyzed. This used to
be the main technique for the purpose, but shooting birds solely to determine diet
is nowadays usually considered unacceptable. The methods for analyzing stomach
contents are similar to those for droppings (10.2.4). The stomach should be
removed as soon as possible and placed in alcohol (high concentrations are prefer-
able as water contents of stomachs may be high) because the contents deteriorate
rapidly. There may be differential digestibilities of prey types and different passage
times, with hard items persisting for longer (Rosenberg and Cooper 1990). Most
of the principles that apply to dropping analysis (see 10.2.4) apply to stomach
contents, remembering that the tendency of the stomach to retain large hard parts
and allow small soft items to pass into the intestines can lead to bias.
In some species, esophageal contents can be quantified and do not suffer from
differential digestibilities (Kundle 1982). Some granivorous species have gullets
or crops in which food is stored before it enters the stomach and again the
contents are unaffected by differential digestion.
Diet composition| 239