Autumn southward migration of dragonflies along the Baltic coast and the influence of weather on flight behaviour

Despite mass movements of insects being documented for decades, whether dragonflies migrate in Europe has not yet been experimentally tested. Similarly, little is known about the influence of weather on the movement decisions and intensity of dragonflies. Taking advantage of large movements of dragonflies along the Baltic Sea coast of Latvia, we investigated whether European dragonflies showed directed movements indicative of migratory behaviour and how weather influences their movements. First, we performed orientation tests with individual dragonflies of two commonly captured species, Aeshna mixta and Sympetrum vulgatum, to determine whether dragonflies showed directed flight and whether flight direction differed from wind direction. Both A. mixta and S. vulgatum displayed a uniform mean southward orientation, which differed from the prevailing overhead wind direction, indicating migratory behaviour. Second, we investigated the influence of weather conditions on the abundance of dragonflies captured. Differences in flight behaviour in relation to weather conditions were observed between A. mixta and the two smaller Sympetrum species (S. vulgatum and S. sanguineum). Generally, temperature, cloud cover and wind direction were the most important predictors for dragonfly abundance, with temperature positively, and cloud cover negatively, influencing abundance. Aeshna mixta appeared to select favourable tail winds (northerlies), whereas abundance of Sympetrum increased with more easterly winds. Our results provide important information on the influence of local weather conditions on the flight behaviour of dragonflies, as well as evidence of dragonfly migration along the Baltic coast.