In the still of the night: revisiting Eastern Whip-poor-will surveys with passive acoustic monitoring

Recent advances in passive acoustic monitoring warrant the review of survey protocols because passive acoustic monitoring can increase sampling effort with minimal additional cost. In particular, protocols for nocturnal species should be re-evaluated because automated processing with signal recognition is expected to perform well for these species and surveys conducted by human observers are often limited by safety concerns. We revisited the best survey practices for the Eastern Whip-poor-will (Antrostomus vociferus), a nocturnal species of conservation concern. Whip-poor-will surveys are typically limited to nighttime, but also to times of high lunar illumination because their calling rate is associated with moonlight levels. We used automated recognition to extract Whip-poor-will detections from a dataset of autonomous recording unit (ARU) recordings from sites with known Eastern Whip-poor-will occupancy in Ontario, Canada. Temperature and time relative to sunset had particularly strong quadratic effects on detectability, with detectability maximized at 13 °C and 4 hours after sunset. Moon altitude and day of year had positive effects on detectability, while wind speed had negative effects on detectability. We found constraining surveys by optimal values of those detectability covariates was worthwhile only up until 10 recordings, at which point the cumulative probability of detecting an Eastern Whip-poor-will at each site was equal between constrained and unconstrained nocturnal recordings. The number of recordings required to reach an asymptote for detectability was between 81 and 97, depending on recording length. We provide objective-specific recommendations for Eastern Whip-poor-will surveys and suggest unconstrained passive acoustic monitoring as the preferred survey method for many objectives. Given the rise of passive acoustic monitoring, survey practices for many species should be revisited because the increases in sampling effort provided by ARUs can improve cumulative detection probability and potentially outweigh the advantages of limiting surveys to times and dates of optimal detectability.