A Sightability Model for Bighorn Sheep in Canyon Habitats

Abstract

Visibility bias (failure to observe all animals) encountered during aerial surveys produces biased estimates of population parameters. Factors affecting visibility during helicopter surveys of bighorn sheep (Ovis canadensis) have not been quantified. We measured visibility bias for helicopter surveys of bighorn sheep in southwestern Idaho. Visibility was influenced (P < 0.05) by activity, habitat, sex composition of groups, light condition, position of sheep relative to the helicopter, and topographic position but not by group size (P = 0.781). Multivariate regression indicated that activity (P < 0.001) and habitat (P < 0.002) variables were the most important factors affecting visibility. A sightability model was developed to estimate bighorn population and composition parameters from data collected during helicopter surveys. We conducted 12 surveys in southwestern Idaho. The estimated population observed during helicopter surveys ranged from 51.7 to 78.1% and averaged 67.1% (CV = 10.6%). Confidence intervals for population estimates ranged from 16.4 to 22.9% and averaged 18.5% (CV = 16.0%) of the population estimate. We recommend correcting survey data for visibility bias to estimate bighorn sheep population parameters.