By the Moonlight Shadow: Examining the Acoustic Ecology of the European Nightjar (Caprimulgus europaeus) in Northern Greece Using Passive Acoustic Monitoring

The European Nightjar (Caprimulgus europeaus) is a Western Palearctic bird known in Europe for its distinct breeding vocalizations during summer nights. It migrates from sub-Saharan Africa, where it overwinters, to reach its breeding grounds in Europe around early to mid-April. Although nightjar ecology has been studied in some parts of its summer range (e.g. Great Britain, Belgium), the species remains unstudied in southeastern Europe. Our aim was to use passive acoustic monitoring (PAM) to study the seasonal and temporal calling activity of the species at two sites in northern Greece (Mt. Chortiatis) across an entire breeding season, examining whether variances in the daily calling frequency could be explained by environmental variables (wind, temperature, air humidity, precipitation, moon phase, cloud cover, night length). We deployed three AudioMoth acoustic sensors, scheduled to record at 48 KHz sampling rate one out of every ten minutes, at each site from 15/4/2024 to 12/10/2024. We used the BirdNet algorithm to detect the minutes with nightjar calls (recall rate 82%). The relation of environmental variables to the daily number of minutes containing nightjar calls was assessed using generalized mixed effect models (GLMMs). They are mostly consistent with previous studies on the calling activity of the species. Calling commenced in late April and continued with decreasing frequency until early September, with variations in onset and seasonal duration across sites. Most calls occurred after dusk and before dawn (crepuscular activity), with calls taking place throughout the night during peak calling period (mid-May – mid-June). Calling frequency was higher during humid, windless, moonlit nights without precipitation and shorter duration. The results provide a baseline against which to examine possible effects of climate change on the breeding behavior of Afro-Palearctic migrant bird species, and we therefore suggest that similar studies, ideally on a continental scale, should be conducted for more species, in order to detect early climate-induced ecological changes.