Conventional Approaches and Their Limitations Direct Observation

Direct observations have been widely applied to document the forage or prey used by a variety of species. Individual animals or groups are observed through binoculars as they graze or feed on an animal carcass. Observations at bird nests also have provided information on foods brought to juveniles (Errington 1932; Marti 1987; Bielefedt et al. 1992). The basic approach is simple and relies on limited equipment. For researchers studying herbivores, bite counts or feeding minutes by plant species are recorded. These values can then be translated into relative occurrence in the diet by comparing total bites or minutes of foraging and the contribution of each species to the total observed. Biomass consumed can be approximated by estimating the average mass per bite for each species incorporated in the diet (Smith and Hubbard 1954). Additionally, direct observations are useful in identifying differences in foraging among sexes or age classes (Illius and Gordon 1987). Unfortunately, this technique is hampered by several limitations (table 5.1). Observations are usually limited to species that occupy open habitat (grassland, savanna, or tundra) and forage during daylight periods. Consistently identifying the grasses and forbs consumed by herbivores may limit the application of this technique to habitats where forage diversity is limited or species are easily differentiated. For wideranging animals, the observer must follow subject animals and thus may affect their foraging behavior or prey availability (Mills 1992). Among carnivores, observations are usually limited to identifying large prey that are not eaten whole (Schaller 1972; Mills 1992).

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