Why do Dragonflies sometimes appear in large swarms?

Several species of dragonfly are known to collect in large aggregations or swarms. In Europe, the Migrant Hawker (Aeshna mixta) and the Four-spotted Chaser (Libellula quadrimaculata) have been observed to do this. In most cases this appears to be due to very favourable feeding conditions in the locality. It may also be a "courting" group with males actively searching for females. This is less likely as males are much more aggressive to each other when looking for a mate. The Four-spotted Chaser occasionally collects in these large aggregations before making a mass movement to another locality (like a bird migration). The reasons for this are unclear but may be due to population pressures. There are records from the US of migratory assemblages of species such as the Green Darner (Anax junius) and various species of Saddlebags (Tramea).  more