In the twilight zone: patterns in Common Nighthawk (Chordeiles minor) acoustic signals during the breeding season and recommendations for surveys

Abstract

Surveys optimized to coincide with peak detectability of target species are critical to the success of monitoring programs, especially those targeting species of conservation concern. Established species-specific survey protocols are often inconsistent between jurisdictions, with limited spatial and temporal data to inform survey timing. The recent proliferation of programmable autonomous recording units (ARUs) and automated detection software enables the processing of huge volumes of acoustic data, which can improve our understanding of the acoustic phenology of many bird species. In May–July 2014, we deployed ARUs across a gradient of latitude near the northern limit of the breeding range of the Common Nighthawk (Chordeiles minor), a species of conservation concern, to quantify variation in temporal detection patterns. Most activity occurred after sunset and before sunrise, with a pronounced peak during civil twilight. We found considerable latitudinal differences in the activity patterns of birds, related to variation in the occurrence or duration of twilight periods. At northern sites (> 60° N), birds were active from dusk until dawn, likely because civil twilight lasted the entire period. At southern sites ( 50° N) and between mid-June and mid-July further north, given high activity rates throughout the breeding season. Given that non-vocal booms are more strongly associated with breeding activity and nesting sites, future surveys should focus on targeting this acoustic signal. Considering the timing of activity patterns in this species, we recommend a targeted, species-specific survey to ensure documentation of their abundance and distribution. Finally, we provide recommendations to improve survey timing and provide advice for acoustic data management and processing in relation to this species