Migrating hoverflies as potential food source for co-migrating insectivorous birds.

Most migrating birds must replenish energy reserves during migration. Food availability significantly influences migratory routes and can even force migrants to detour, but still little is known about potential co-migration between insectivorous birds and their insect prey. To address this gap, we focused on day-flying insects and the insectivorous birds migrating through the Červenohorské sedlo mountain pass, Czech Republic. During four seasons of insect and bird trapping, using Malaise trap and mist-nets, respectively, we recorded 23 094 birds of 80 species and 35 087 migrating hoverflies (Syrphidae) of 47 species. We found a strong temporal correlation between the number of migrating hoverflies and insectivorous birds crossing the mountain pass. The observed pattern suggests that a similar phenomenon may occur in lowlands, where both groups stop over before and after crossing the mountains. These stopovers may provide migratory birds with abundant and reliable food resources. We also found that hoverflies comprised 88% of the biomass of all trapped insects, making them the most abundant potential prey of migrating birds. Our results outline the co-migration of birds and hoverflies and shed light on possible predator-prey dynamics during migration.