Detections of burros from helicopter aerial surveys in the southwestern US, 2016-2018

Feral burros (Equus asinus) and horses (E. ferus caballus) inhabiting public land in the western United States are intended to be managed at population levels established to promote a thriving, natural ecological balance. Like many large ungulate populations, management agencies employ aerial surveys to obtain estimates of horse and burro population sizes. Double-observer sightability (MDS) models perform well for estimating feral horse abundances, yet the effectiveness of these models for use in burro populations is less understood and may be different due to the smaller size, stoic behavior, and cryptic pelage of burros. These models help minimize detection bias, yet bias can be further reduced with models that account for unmodeled variation, or residual heterogeneity, in detection probability. In populations containing radio-marked individuals, residual heterogeneity can be estimated with MDS models by including an additional capture history corresponding to detections of marked individuals because these detections provide information on groups missed by both observers. One approach to estimate residual heterogeneity is including a covariate in detection models that estimates the difference in detection probability between marked and unmarked groups. Such a difference relies on many marked groups having no probability of detection; but this may not be realized in all populations, and therefore may underestimate residual heterogeneity. Another approach is to use information from detections missed by both observers to account for the characteristics that make groups more or less likely to be detected, or "recaptured", by the second observer. This dataset contains burro detections from aerial surveys conducted between 2016-2018 in 3 burro populations that were used to develop MDS models applicable for feral burros in the southwestern United States. We also include an R script that will help users set up and run 4 types of MDS models: a standard MDS model without radio-telemetry, an MDS model with telemetry that assumes no residual heterogeneity is present; a MDS model with radio-telemetry that estimates residual heterogeneity by the difference in detection between marked and unmarked groups, and an MDS model that accounts for residual heterogeneity through recapture probability.