Environmental drivers of bat activity at high latitudes.

In high-latitude ecosystems, insectivorous bats must make use of short nights during the summer, during which temperature and light conditions interact in non-linear ways. These abiotic dynamics directly influence insect availability and, in turn, bat activity. We examined the influence of night-time temperature and night length on the nightly and seasonal activity patterns of the northern bat (Eptesicus nilssonii) across Finland (60°–66°N), using passive acoustic monitoring data collected over 9,000 detector-nights across seven years. We predicted that bat activity would be constrained by low temperatures, strong winds or heavy rain, and that the nightly pattern would depend on an interaction between night length and weather conditions. Our results show activity peaked 1–2 h after sunset under most conditions, with ambient temperature strongly influencing overall activity levels. On short, warm nights, activity was relatively evenly distributed across the few hours of darkness, whereas long, warm nights produced a bimodal activity pattern with peaks after sunset and before sunrise. This dual-peaked pattern occurred only under thermal conditions that likely support sustained insect activity. In contrast, on colder or windier nights, activity was sharply concentrated to the first hours after sunset. Seasonal and latitudinal trends revealed that activity was most restricted in spring, particularly in northern regions, while the progressing summer expressed more evenly distributed patterns. In autumn, activity patterns diverged across latitudes, reflecting interactions between temperature and night length. The results of our study demonstrate that E. nilssonii may dynamically adjust its foraging behaviour in response to interacting abiotic constraints, optimizing energy gain while minimizing predation risk. Our findings quantify the environmental thresholds facilitating various activity patterns and provide rare evidence of bimodal nocturnal activity at high latitudes. Our study highlights the importance of large-scale, long-term monitoring for understanding how changing climatic conditions influence species behaviour in boreal ecosystems.