Abstract
High altitude mountain passes in the Pyrenees are known to be important migratory hotspots for autumn migrating insects originating from large swathes of northern Europe. In the Pyrenees, prior research has focused on diurnal migratory insects. In this study, we investigate the nocturnal component of the migratory assemblage and ask if this transient food source is also used by bat species. Three seasons of insect trapping revealed 66 species of four different orders, 90% of which were Noctuid moths, including the destructive pest Helicoverpa armigera, otherwise known as the cotton bollworm. Acoustic bat detectors revealed that high activity of Nyctalus spp. and Tadarida teniotis bats were closely synchronized with the arrival of the migratory moths, suggesting this food source is important for both resident and migratory bats to build or maintain energy reserves. Bats of the Nyctalus spp. are likely migrating through the study site using fly-and-forage strategies or stopping over in the area, while resident T. teniotis may be exploiting the abundant food source to build fat stores for hibernation. This study shows that nocturnal migratory insects are abundant in the Pyrenees during autumn and interact during migration, not only with their co-migrant bats but also with resident bat species.