How Passive Acoustic and Traditional Monitoring Estimate Bird Diversity Across Habitats

ABSTRACT

Bird monitoring techniques have evolved substantially in recent years. While the most common protocols remain the point count and transect survey, new technologies such as passive acoustic monitoring (PAM) are becoming increasingly popular. As more monitoring programs adopt PAM, it is crucial to understand how comparable it is with classical monitoring derived data. We compared diversity indices (i.e., species richness, Shannon diversity and Jaccard index) derived from both classical (by means of observer‐based point counts) and passive acoustic (by means of recording devices) monitoring and developed a novel estimated Shannon index based on PAM, using species‐specific confidence thresholds and detection rates. We applied the protocols at 126 sites in a heterogeneous landscape within the Italian Alps, enabling us to investigate for the first time whether the effectiveness of different monitoring schemes depends on the habitat type. We show that PAM captured approximately twice as many species as the classical point count method. The species richness estimated by both protocols was strongly correlated (r _s  = 0.71), but with major differences between habitat types. The difference was greater in diversity hotspots such as wetlands and meadows and lower in alpine habitats. The novel PAM‐based Shannon index was very reliable and yielded similar results to the observer‐based Shannon index, illustrating its effectiveness. Species composition differed significantly between the two protocols (average Jaccard index = 0.38). Although this study highlights the additional information that PAM can provide, bird counts remain essential as they offer further insights (e.g., breeding status) and not all species can be captured by PAM (e.g., acoustically indistinguishable). As point counts are time‐ and cost‐efficient, they will continue to play an important role in bird monitoring. Our results highlight the strengths and weaknesses of each approach and provide recommendations for future monitoring programs.