A standard method of assessing fruit quality is “relative yield”; that is, the dry
mass (or better still ash-free dry mass) as a proportion of the total mass (Snow and
Snow 1988). If necessary, the fruit pulp can be analyzed for its carbohydrate, fat,
and protein content. Seed mass as a proportion of the whole fruit mass is a use-
ful measure of the inedible weight that has to be carried if the fruit is swallowed.
10.4.2 Prey digestibility
The digestibility of a given food type is usually determined using captive birds.
It can be assessed by providing a known mass of food f, then collecting all the
droppings and determining their dry mass b. The droppings can be washed off
a plastic sheet or tray.
The digestibility dcan then be calculated as
db/f.a
where ais the fraction of the mass of the fresh food retained when dried.
Digestibility can also be assessed in the field in some circumstances using an indi-
gestible marker. The amount of indigestible marker Miand the amount of energy
(measured by microbomb calorimetry), protein Cior carbohydrate or any other
component of the diet is measured in the food and in the droppings (MdandCd).
The digestibility dcan then be calculated:
In the past cellulose was often used, but it appears to be digested (Buchsbaum
et al. 1986). Lignin is less readily digested and is probably preferable, but large
species digest a higher proportion of such fibers. Trace elements like magnesium
can also be used if the bird can be assumed to be in balance for them.
The food analyzed should contain the same proportion of marker and protein
as the samples eaten. Birds are often highly selective for samples high in protein
but low in fiber, such as young leaves, so analyzing a mix of young and old leaves
would underestimate digestibility.
10.5 Foraging behavior
10.5.1 Time budgets
It can be useful to assess the time spent on different activities. This may involve
comparing the time budgets of different individuals, ages or sexes, or birds at dif-
ferent locations. The two main methods are focal sampling and scan sampling.
Focal sampling consists of watching an individual for a fixed period (e.g. 10 min)
dCiMd
CdMi244 |Diet and foraging behavior