How to better count elusive birds? Comparing non-invasive monitoring methods to estimate population size of the threatened Pin-tailed sandgrouse (Pterocles alchata)

Abstract Abundance is essential to monitor population trends, assess viability, and determine the conservation status of species. Genetic mark-recapture (GMR) has become a popular non-invasive tool for abundance estimation, overcoming the challenges posed by the low densities and sometimes elusive behaviour presented by threatened species. Steppe birds are one of the most threatened bird guilds in Europe, but their populations remain understudied. Although GMR is widespread in mammals, few studies use GMR to estimate abundance of bird populations, which are usually monitored via direct observation. To date, there are no studies comparing GMR with other common approaches for bird monitoring – such as Hierarchical Distance Sampling (HDS) – to estimate bird abundance. Here, we compare one GMR and two HDS monitoring approaches to estimate abundance of the threatened Pin-tailed sandgrouse ( Pterocles alchata ) in the year 2022 in the Lleida Plain (Spain). We developed the first set of molecular markers for this species, which provided a high genetic identification success. This resulted in GMR yielding the most precise abundance estimates. The HDS approach using a general survey design for farmland bird monitoring provided inaccurate abundance estimates and required integrating data from previous years. The HDS approach mimicking the Pin-tailed sandgrouse national monitoring program resulted in a low number of detections, which hampered abundance estimation. We evaluated pathways to optimize GMR costs and provide sound recommendations for monitoring. Our results showcase the promise of GMR for monitoring Pin-tailed sandgrouse, and highlight its potential to assess the conservation status of small populations of steppe bird species.