Bioacoustics as a Measure of Population Size and Breeding Success of European Storm Petrels <i>Hydrobates pelagicus</i>

Obtaining measures of population size and fitness are key first steps to understanding how and why species' populations change over time. Quantifying such metrics is difficult in some species, however, due to their remote location and/or ecology, that is they may be widely dispersed or may not be readily monitored visually. As such, bioacoustic monitoring is increasingly used to monitor populations of such species, as in burrow-nesting seabirds. However, while a growing number of studies successfully obtain measures of population size using bioacoustics, there are few that effectively quantify measures of population fitness, limiting the conservation value of this tool. Here, we investigated whether bioacoustics could yield indices of population size and a key population fitness measure, breeding success, comparable to those derived from observer-based methods in a breeding population of a burrow-nesting seabird, the European Storm Petrel, <i>Hydrobates pelagicus</i>, on Mousa, Shetland. We used AudioMoths (Open Acoustic Devices) to record storm petrel adults and chicks over a 12-week period from June to August 2023 and concurrently undertook observer-based surveys. We then used a classifier model to quantify the average nightly call rate of adults and chicks across the recording period as bioacoustic-derived measures of population size and breeding success, respectively. We found that observer-based and bioacoustic-derived measures of population size were significantly positively related for the two types of adult calls. Further, we found that observer and acoustic measures of chick abundance had a significant positive relationship, and a weaker, yet still significant, relationship for breeding success. Consequently, we demonstrate the utility of bioacoustics to provide relative measures of population-level parameters and provide recommendations for future research. Bioacoustic monitoring can provide a method to monitor colonies while requiring substantially less time in the field, and so may facilitate more regular and comprehensive monitoring at colonies of burrow-nesting seabirds.