Hybrid mark–resight approaches account for heterogeneous detection bias in aerial surveys

Abstract

Population abundance estimation is a critical yet challenging task for wildlife management decision‐making. For aerial surveys of terrestrial mammals in highly heterogenous landscapes, estimation models that address multiple sources of detection bias are needed. A typical hybrid mark–resight model includes an additive parameter that accounts for detection probabilities derived from marked animals. While this additive hybrid method improves abundance estimates, there remains a need to account for heterogeneity due to the conditional dependence in detection probability inherent in aerial surveys with multiple observers. We propose a modified approach, referred to as a conditional hybrid model, to address this heterogeneity directly. The conditional hybrid model applies a mark–resight approach typically used to account for tag loss in mark–recapture studies to aerial surveys with a subset of the population marked with GPS collars. We conduct a structured comparison between the additive and conditional hybrid estimation models and fit these two hybrid models to data from a 2020 aerial survey of mule deer in southern California. Both the additive and conditional hybrid models had general agreement in overall model structure and variable importance, however, the conditional model outperformed the additive model in capturing sources of detection heterogeneity and overall abundance estimates differed by 9%. Our top‐ranked models suggest that deer movement, larger group sizes, and rougher topography increased detection probability, whereas high cloud cover, obscuring vegetation and high observer fatigue and helicopter height decreased detection probability. Our findings illustrate the need to assess multiple correction methods when estimating abundance from aerial survey data to ensure accurate abundance estimates are being used to inform wildlife management strategies, with the caveat that specific survey conditions may dictate which method is the most appropriate for achieving robust estimates. These approaches represent the cutting‐edge science on wildlife abundance estimation from aerial survey data, a critical ingredient for effective wildlife management. By testing innovative hybrid methods, we present the first formal comparison of the strengths and weaknesses of each method within a highly heterogeneous landscape, providing practitioners with necessary information for future application.