Diel and seasonal vocal activity patterns revealed by passive acoustic monitoring suggest expert recommendations for breeding bird surveys need adjustment
D. Singer, Johannes Kamp, Hermann Hondong, Andreas Schuldt, Jonas Hagge
Abstract
Abstract Species identification and recording in breeding bird surveys vastly rely on the registration of avian calls and songs. Despite comprehensive expert knowledge on species-specific activity patterns, data-based analyses of vocal activity patterns are lacking. Recent advances in passive acoustic monitoring allow the direct measurement of bird vocal activity at very high temporal resolution. We conducted a comprehensive survey, recording 25,000 h of audio data at 256 forest sites in Lower Saxony, Germany, to investigate vocal activity patterns of the European forest bird community. Our results reveal a high degree of inter-specific variability in seasonal and diel vocal activity patterns, including strong circular patterns along the day–night cycle and a significant seasonal component. Comparing acoustic detectability to species-specific survey recommendations revealed critical temporal discrepancies for 64.2% of species, and standard protocols (sunrise to 4 h after sunrise) showed discrepancies for 41.5% of species. This highlights the potential for temporal survey optimization to reduce imperfect detection and increase accuracy and precision. Emphasis should be given to the hours before and after sunrise and also sunset for sampling less detectable species. Combining observer-based surveys with passive acoustic monitoring might leverage the strengths of both methods. Our results also emphasize the potential of continuous recording schedules in passive acoustic monitoring to capture diverse temporal patterns. This study provides a baseline for future research on vocal activity patterns across habitats, throughout the year, and regarding anthropogenic impacts. Our findings may raise awareness among ornithologists about the sources of variation in acoustic detectability and its implications for breeding bird surveys, highlighting potential for methodological adjustments in survey timing and consequences for carful interpretation of bird surveys.