Hybrid bioacoustic and ecoacoustic analyses provide new links between bird assemblages and habitat quality in a winter boreal forest

Resident birds in boreal forests can serve as indicators of habitat quality and are often species of conservation interest, particularly in multifunctional forests also used for timber production. To make informed forest management decisions, we must first understand which structural features provide habitats useful for resident birds. This is particularly true in winter, an understudied and critical season for their survival. The objective of this study was to establish reliable methods for monitoring bird presence and activity during winter, and to use these methods to evaluate the relative importance of stand structural features to make inferences about which features support and increase winter survival potential. Using a hybrid bioacoustic and ecoacoustic approach, we tested the ability of acoustic recordings to identify links between bird diversity and components of structural complexity, and compared these results to those from the traditional point count method. We conducted a vegetation survey, point count surveys and collected acoustic recordings from December 2019-February 2020 in 19 sites in a Swedish boreal forest. First, we compared species richness values derived from point counts and bioacoustic monitoring methods. Bioacoustic species richness was significantly higher than point count richness, although only when the time spent identifying species from recordings exceeded the time spent conducting point counts in the field. Next, we demonstrated that bioacoustic species identification yields additional metrics of bird activity that point counts cannot. We tested the response of these metrics, and point count metrics, to variables of structural heterogeneity and complexity of our sites. Almost all bioacoustic metrics increased significantly with increasing structural complexity, while point count richness and abundance did not, indicating that automated recording is more effective in identifying forest patches of high quality in winter. Lastly, using an ecoacoustic approach, we calculated six of the most common acoustic indices and tested if any could effectively reflect the bird-structure relationships described above. Two indices showed significant positive relationships to bioacoustic metrics, demonstrating their potential as biodiversity assessment proxies that respond to differences in habitat quality. This is the first winter acoustic study to monitor bird assemblages in detail; it employed both bioacoustic and multi-index ecoacoustic approaches, which provided evidence that automated acoustic recording can be an effective and superior method for monitoring resident forest birds.