Estimating Owl Population Density Using Acoustic Spatial Capture-Recapture

Acoustic spatial capture-recapture (SCR) methods appear ideally suited for estimating population trends of cryptic species that are difficult to monitor. Despite evidence suggesting acoustic SCR offers greater precision over estimates obtained through distance sampling or mark-recapture approaches, it has seldom been applied to avian taxa. Here we assess the utility of acoustic SCR for estimating population density of owls, a group of birds for which population estimates are generally lacking. We conducted passive acoustic surveys for Barred Owls (Strix varia) and Great Horned Owls (Bubo virginianus) over the spring breeding seasons of 2020-2021 in a protected park in Georgia, USA. We used a combination of acoustic discrimination techniques to identify individuals, including spectrogram cross-correlation, hierarchical clustering, and visual scanning. The resulting estimates of the density of Great Horned Owl individuals (0.28 6 0.026 [SE] per km2) were consistent with population density estimates from previous studies in other parts of the species' range. However, our methods appeared unsuitable for distinguishing Barred Owl individuals, which prevented estimation of population density for this species. We provide recommendations to improve acoustic sampling and individual discrimination of owls and suggest that acoustic SCR offers important advantages over conventional methods for monitoring some cryptic birds.