10.2.2 Nest observations
The food carried in the bill to the nestlings or incubating partners can often be
identified but may differ from that taken by the foraging adults themselves.
Foraging adults often swallow small items they find, but take large prey back to
the nest.
Watching from a hide overlooking the nest can be useful. Excellent data on the
diet of chicks can be obtained using nest cameras with an infrared beam fixed so
the bird triggers the camera as it returns to the nest. This equipment is available
commercially. For birds using nest boxes, the camera can be placed inside so that
entering birds are photographed. The camera obviously needs a motordrive and
some can be adapted to take long series of exposures (e.g. 250). Slide film is
probably easiest for subsequent identification; an alternative is to use a video.
The camera can be hidden within a box and a car battery used as a power supply.
If a clock is placed in view of the camera, then nest daily patterns and provision-
ing rates can be accessed. Many cameras can print the date and time of exposure
onto every frame. A ruler can be placed at the same distance so that prey size can
be estimated from the photographs. Alternatively the prey can be related to bill
length. Combining data from nests with observations of foraging behavior
means that the diet can be linked to the feeding habitat. Analyzing films and
especially video can be tedious, so consider the time that this will take before
starting the study and allow for the fact that the camera may not work well all the
time. Observations are needed to see whether this apparatus affects the behavior
of the birds.
10.2.3 Remains and signs
Birds often leave evidence of food items they have taken. For herbivorous species
look for bite marks in vegetation, while for seed-eaters look for discarded husks.
The remains of vertebrates that are not eaten whole can be examined to deter-
mine species, age, sex, condition, or parasite load (see Chapter 8). Vertebrates
captured by mammals often show bites on the bones and can thus be distin-
guished from those captured by birds (which may have āvā shaped pieces missing
from the edge of the sternum). Eggs preyed upon by birds often contain a pool of
yolk as they cannot lick the remains clean. Some species remove heads or wings
of insects or leave the shell of molluscs and crustaceans. Observations are neces-
sary to see what proportion of items are swallowed whole. The number of leaf
miners removed by insectivorous birds can be determined by the presence of tear
marks on the mine. Crossbills, woodpeckers, and squirrels all tackle tree cones
in different identifiable ways. Collecting and identifying pollen from the
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