Evaluating two avian monitoring tools for detecting relative abundance of species during autumn migration

Detecting changes in abundance through time is important for understanding how factors like climate change or habitat conditions affect migrating populations and is limited by our ability to monitor them. We compared two widely used monitoring tools to determine whether they are comparable for detecting relative abundance of migratory birds. This study evaluated how flight calls detected by autonomous recording unit throughout the night compared to diurnal detections of birds captured via standard effort mist netting the following morning at three elevational sites in the Intermountain West. Across all species, mist netting performed better for characterizing species diversity. Among the 21 species detected at least once by both methods, increased detection of nocturnal flight calls was significantly (p = 0.036) but only moderately correlated (r = 0.46) with increases in mist-net captures the following day, demonstrating that the two methods are not adequately equivalent for tracking relative abundance. Each method has strengths as well as limitations, and the target species or project goal may dictate which monitoring method should be applied.