The estimation of large-ungulate population size involves a systematic search of occupied habitat and visual observation of individuals. In this study we determined the probability that forward-looking infrared radiometer (FLIR) mounted in a fixed-wing airplane would detect and verify California bighorn sheep (Ovis canadensis californiana). The study area included the highly dissected rhyolite canyons of southwestern Idaho. All age and sex classes could be detected with the FLIR. Flying at 600 m above ground level (AGL), FLIR could distinguish bighorn sheep from other ungulates and large mammals (i.e., pronghorn [Antilocapra Americana], mule deer [Odocoileus hemionus], livestock, and mountain lion [Felis concolor]). Image clarity and the ability to circle the animal without disturbance allowed determination of male age classes for use in setting harvest of available rams. Bighorn sheep could be detected in all habitats used within the study area. Data were collected over 3 years, with probability of detection of 89%. A set search pattern allowed consistent detection rates between sensor operators, airplane type, or among years. This study identified variables that influence sighting probability using FLIR. The use of a FLIR mounted on an airplane flying at 600 m AGL has advantages over surveys using visual observations from airplanes or helicopters; those advantages include reduced stress to the animals, reduced violations of assumptions of sightability models, and reduced hazard to observers.
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