prey available. For example, observations may show that the bird is feeding from
leaf litter in damp areas in the forest and taking long thin items about the same
length as the bird’s bill. Sampling beneath the leaves in the same habitat shows
the only two prey groups that both occupy this habitat and fit this description are
leeches and worms. Further observations will often distinguish between them
due to differences in color, shape, or predator behavior (e.g. leeches can be picked
up but worms may have to be pulled from their burrows). Alternatively, tech-
niques such as dropping analysis can then be used to make the final assessment.
Because analysis of droppings relies on the identification of food fragments that
are resistant to digestion, direct observation is particularly useful for seed-eating
birds that remove the hard seed coat (testa) before eating the seed and birds that
feed on invertebrates with few hard parts.
On muddy areas it is sometimes possible to examine footprints and peck
marks to see where the bird pecks. This can sometimes be used to see if it pecks
at features such as holes in the mud or to link the footprints to food remains.
Studying habitat choice can facilitate an assessment of diet. Discovering that
the species prefers feeding on one patch or one tree species, or feeding in an area
at a particular time of day, especially if accompanied by examination of potential
food items.
The usefulness of direct observations varies. Utilization of seeds and fruits taken
direct from the plant can often be determined unambiguously. The diet of birds on
mudflats is often easy to determine from observation and mud sampling, because
there are relatively few prey species and they differ markedly in appearance. Even
for species where direct observation seems impossible (e.g. such as canopy
dwellers), it is useful to know if they take food from tree trunks, underneath leaves,
or if they catch prey in mid-air. In most cases, observation of feeding methods and
feeding places can narrow the range of possible items but other techniques are neces-
sary to fully determine the diet. Food hoarders, such as many tits (chickadees) and
corvids, may appear to be foraging when actually searching to retrieve cached
items. Finding the caches provides a way for the observer to record the food.
Taking captive-hatched tame chicks of precocial species to selected locations
in the field and observing what they eat may seem a useful way to study the diet.
However, this method should be used with caution. In many species, such as
galliforms, the mother draws the attention to specific types of prey items by picking
them up in the bill and dropping them and uttering special calls (“tit-bitting”)
and may also make prey available to chicks by scratching them up from the litter
layer or knocking down insects from tall vegetation. This behavior by the mother
probably has a large influence on what the chicks eat and may make the diet of
captive-hatched chicks feeding on their own a poor guide to that of wild chicks.
234 |Diet and foraging behavior