A new method to estimate abundance of Australasian Bittern (Botaurus poiciloptilus) from acoustic recordings

Effective conservation management relies on survey methods that accurately represent the biological communities being monitored. Here, we describe a novel approach using long-duration acoustic recordings to estimate abundance of a threatened wetland bird, the Australasian Bittern (Botaurus poiciloptilus). Whereas acoustic monitoring enables a large increase in effort compared to traditional on-site monitoring, e.g., triangulation surveys, it is difficult to estimate the number of individuals of a target species in acoustic recordings. We describe a semi-automated approach to estimate bittern abundance at four sites in the Barmah-Millewa Forest of southern Australia using single-channel, long-duration recordings. Our approach leveraged several known characteristics of bittern calling behavior. We obtained abundance estimates that are larger than those previously found using triangulation surveys at the same site. This is primarily attributed to our ability to find the peak calling hours in a long-duration recording, which does not require the training of a machine-learning call-recognizer. If the method we describe is performed in a consistent, standardized manner, it can identify population trends, which is an important outcome for a threatened species. Our method should be suitable for other furtive wetland species with a similar call structure or frequency range.