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overestimated (Anthony and Smith 1974; Vavra et al. 1978; Smith and Shan-
druk 1979; McInnis et al. 1983). Inaccuracy of the technique, particularly
when applied to diets of mixed (grasses, forbs, and browse combined) feeders,
has led some researchers to question its usefulness for herbivores other than
grazers (Gill et al. 1983). Lack of experience and training in identifying plant
fragments are often cited as the most important sources of error in using the
microhistological technique (Holechek et al. 1982).
In contrast to herbivore samples, fecal material and pellets from carnivores
require minimal preparation. Depending on the size of prey and the level of
mastication, much of the material in a sample from a carnivore may be identi-
fied using field guides to birds, mammals, and insects for comparison. Other
items may be identified through bones, teeth, hair, feathers, or scale patterns.
Therefore, reference materials may include complete skeletons of vertebrates,
samples of hair, feathers, scales of fish and reptiles, and exoskeletons of insects.
A collection of dorsal guard hairs of the mammals likely to be encountered is
particularly useful because of the characteristic features of color banding,
medullary pigment patterns, and the morphology of cuticular scales (Adorjan
and Kolenosky 1969).
Recently, several investigators have used postingestion samples to explore
selectivity of prey consumption or differential vulnerability. These studies were
based on skeletal remains in feces that could be distinguished to different sex
or age categories (Dickman et al. 1991; Koivunen et al. 1996; Zalewski 1996).
Composition of prey remains was then compared to trapped samples. Al-
though this approach has increased the information obtained using post-
ingested samples, it is important to recognize that for larger prey it is not pos-
sible to determine whether the prey was killed or scavenged.
Probably the most contentious aspect of using postingested samples is
the lack of an unambiguous method to quantify the contribution of specific
foods to the total biomass consumed. A variety of approaches have been
used, including frequency of occurrence of specific prey among samples
(Murie 1944; Litvaitis and Shaw 1980; Ackerman et al. 1984), measured
weights of remains recovered in fecal samples (Johnson and Hansen 1979;
Corbett 1989), relative volume of prey remains (Hellgren and Vaughan 1988),
and estimated biomass consumed calculated with digestibility coefficients
(Lockie 1959; Floyd et al. 1978; Greenwood 1979; Johnson and Hansen
1979; Weaver 1993; Hewitt and Robbins 1996). Evaluations of these ap-
proaches relative to ranking prey importance indicated that the most common
inconsistencies occurred among small prey (Corbett 1989; Ciucci et al. 1996;
figure 5.1).