Monitoring of owl populations can be used to assess secondary poisoning hazards to nontarget wildlife from rodenticide use. An indicator species is chosen based on its likelihood to intercept treated areas and target species as well as its sensitivity to the toxic compound. Hazard evaluation includes monitoring of rodenticide-use patterns, radiotelemetry to document nontarget survival or mortality posttreatment, and necropsy and residue analysis. Data on the natural history of the nontarget species (for example, habitat use, foraging behavior) are necessary to interpret the degree of hazard to individuals. When a hazard to individuals is found, data on population dynamics are critical for interpreting the hazard to nontarget populations.