Acoustic monitoring of nocturnally migrating birds accurately assesses the timing and magnitude of migration through the Great Lakes

ABSTRACT Tracking the movements of migratory songbirds poses many challenges because much of their journey takes place at night. One promising technique for studying migratory birds relies on microphones to record the nocturnal flight calls produced by birds on the wing. We compared recordings of night flight calls with bird-banding data in a southern Great Lakes ecosystem. We collected >6,200 hr of nocturnal recordings at 7 locations around Lake Erie. We detected >60,000 flight calls from migratory birds and classified 45,775 calls to species level or to a bioacoustic category comprising several species with similar calls. We compared these acoustic data with records of 5,624 birds captured in mist nets. We found that acoustic recordings accurately quantified the magnitude of migration; comparison with mist-net data revealed significant positive correlations between the number of acoustic detections and the number of mist-net detections across species. We also found that acoustic recordings accurately quantified the timing of migration; we found significant positive correlations between the date of passage of the 10th, 50th, and 90th percentiles of the populations of up to 25 groups of passage migrant species in the acoustic data and mist-net data. A careful examination of 6 species with distinctive flight calls revealed only subtle seasonal differences between peak detections via acoustic monitoring and mist netting, at both daily and weekly timescales. This research enhances our understanding of the role that acoustic sampling can play in monitoring migratory birds, providing important empirical support for the validity of night-flight-call monitoring.