with minimal error. Mouthparts of invertebrates are often hard and relatively easy
to identify so are usually counted and measured, but if mouthparts are hard to
identify, legs might be counted instead.
Since mass (often ash-free dry mass) is a function of volume, it will usually not
be directly related to linear measurements; therefore mass and length are best
logged before calculating a regression equation for this relationship. Calver and
Wasler (1982) suggest which diagnostic parts should be measured for a range of
Hymenoptera, Coleoptera, and Diptera families and provide the regression
equations to link these measures to total length. Morris and Burgis (1988)
give the relationship between fresh weight Wof UK passerines and humerus
lengthH:
lnW3.80272.4221 (ln H)This relationship also provided a good fit for non-passerines with similar body
shape but not for others such as swifts, gulls, and waders, with different relative
leg lengths.
10.4 Prey quality
10.4.1 Energy content
It is often useful to assess the energy content of prey taken. The usual method is
to assess ash-free dry mass as an indication of the energy content. Standard prac-
tice is to dry at 90C until a constant mass is reached. The duration needs to be
tested initially but could be up to several days for larger items, which are there-
fore best cut into pieces beforehand. The prey is then placed in a crucible in a
muffle furnace at 550C until all the organic matter has been burnt off (2 h is
sufficient for most groups) and left in a desiccator to cool before weighing the
ash content. High temperatures cause some conversion of carbonate to carbon
dioxide providing inaccurate estimates for calcium rich species, such as molluscs
and crabs, so it is best to first remove the shell if possible, for example, by placing
molluscs in boiling water for 10 s. The ash-free dry mass is the dry mass minus
the ash content. Measuring the prey beforehand allows a regression of ash-free
dry mass against size to be calculated, so that the measures of prey size captured
can be converted to intake. This can be used to calculate the energy intake from
different prey types, in different areas, or at different times.
To actually measure energy, it is necessary to use a bomb calorimeter to deter-
mine the energy content per ash-free dry mass. This is usually calculated sep-
arately for each prey species or category. Robel et al. (1995) tabulate the energy
and nutrient values of a wide range of invertebrates.
Prey quality| 243