The toxicity values used to derive the avian and mammalian PNECs were based on test organism exposures to highly soluble and, hence, bioavailable forms of Ni. Thus, both the avian and mammalian PNECs are expected to overestimate the bioavailability of biologically incorporated Ni in natural diets. In addition, in the terrestrial pathway, soil-adsorbed Ni in the earthworm gut is expected to have reduced bioavailability. Differences in the bioavailability of a substance between exposure media can be described using a relative absorption factor (RAF), which is the ratio of the absorbed fraction of the substance from one exposure medium versus the absorbed fraction from another exposure medium. The RAF can then be incorporated into the risk characterization, as shown in Section Risk Characterization. RAF values of 3.6% and 2.5% were derived for mammals consuming earthworms and other prey (e.g., isopods), respectively. For birds, no studies were identified on the relative bioavailability of Ni sulfate (the basis for the avian NOEC) added to laboratory diets versus biologically incorporated Ni, and no data on the bioavailability of Ni in soil are available for birds. Consequently, avian dietary and soil RAFs were not derived for birds (i.e., 100% absorption was assumed, which is a conservative approach).