Do Migrant Hawkers migrate in groups?

This summer it appears to be a bumper year for Migrant Hawkers. Their sudden appearance in several places with several individuals present, and the huge numbers seen around makes me think that these are migrants, rather than locally bred individuals. Last week I took a clip of a large group feeding by a nearby wood, a photo would not have conveyed the feeling of watching these dragonflies hunting in groups, paying little notice to one another, in contrast to the two feisty Brown Hawkers in attendance, who squabbled every time they met.

A sunny spell earlier today brought out at least 10 individuals over the garden, the largest number I've had. They are not aggressive or territorial, but, is there more to it? When roosting, they actually appear to choose to rest near other individuals, despite an abundance of sites nearby with a similar aspect, even when breeding (top shot, two mature males basking close to one another at Hornsea Mere, 24 September 2018. 

The following, much more spectacular image was shared on twitter by @jwood_t on the 2nd of August shows 12 Migrant hawkers roosting side by side:

Given that they appear to be a sociaI species, feeding in groups and roosting close to each other, I wonder if they are attracted to one another during migration. Do they actively seek each other, migrating in a compact flock? And why are they social, is there an antipredator advantage, or a better timing and orientation of migration, a quorum decision on directions, or even an energetic advantage, when migrating in groups?

Migration in groups would not be a new phenomenon in dragonflies. A striking example is this amazing photo of migrating dragonflies shared by Dave Smallshire:

My final offering for #worldodonataweekend @Britnatureguide @BDSdragonflies: a unique image taken high over Gomatong Cave, Borneo showing a world first - 2 of about 10 migrating flocks of dragonflies! These can be identified as Rhyothemis obsolescens from their wing pattern. pic.twitter.com/zLlUez9jEV

— Dave Smallshire (@smallshire) May 10, 2020

The Migrant Hawker is indeed known for migrating in large swarms - hence its name - but little experimental research has been carried out on this species. Recent research conducted at a bird observatory on the Baltic coast in Latvia starts to address this gap. Aline Knoblauch and collaborators took advantage of dragonfly 'by-catch' in Heligoland traps during a few weeks in August and September. These large funnel-like traps covered on wire-netting, widely used to investigate bird migration, also capture dragonflies, unharmed. The researchers also used field 'orientation experiments, releasing freshly caught dragonflies into a closed round arena made of mesh and analysing recorded videos to determined if they had a preferred flight direction. All dragonflies were released after the short trials. Their results showed that individuals captured during autumn migration orient themselves to a southerly flight direction irrespective of the prevailing winds, even though there were more captures in the Heligoland trap when the prevailing wind were northerly wind, indicating that the dragonflies were choosing to fly when wind direction was favourable. These results indicate true migration, rather than random accumulations of feeding individuals in suitable areas. This experiment, however, doesn't answer the question of orientation to other individuals.

An observational study on the related species, Southern Migrant Hawker, Aeshna affinis, provides some intriguing clues about group migration, likely to apply to Migrant Hawkers as well: 

"In the late afternoon, thousands of immature individuals were flying above the swamp, mainly at a height of 5-10 m. They formed a huge compact bubble, which was spatially clearly defined on the outside, but with unorganised bee-like swarming inside. Individuals showed a distinctly jerking and dancing flying style, with a minimum space kept between individuals."

This indicates not just agglomeration due to being in the same area with the same intent of migrating, but active flock formation behaviour. The insects also appeared to follow geographical features, a river, to migrate. 

Another interesting feature of Migrant Hawker migration has been noted in a study recording the numbers of Migrant Hawkers migrating on the Danube delta. A strong male-bias of 2 males to 1 female was found, which was also noted in the first study. Although sex-biased dispersal and migration is well known in many animals, it is unclear if this is due to different timing of migration of males and males, or to different geographical or altitudinal pattern of migration of sexes. Note that observational studies are limited to dragonflies flying low, and Heligoland traps only capture dragonflies flying just above ground (within about 3 m).

We are starting to understand Migrant Hawker migration, but there is still a lot of questions to answer.

More information

Knoblauch, A., Thoma, M. & Menz, M. H. M. Autumn southward migration of dragonflies along the Baltic coast and the influence of weather on flight behaviour. Anim. Behav. 176, 99–109 (2021).

Schröter, A. A mass migration of Aeshna affinis in southern Kyrgyzstan: attempt to provide a spatial and temporal reconstruction (Odonata: Aeshnidae). Libellula 30, 203–232 (2011)

Dyatlova, E. S. & Kalkman, V. J. Massive migration of Aeshna mixta and Sympetrum meridionale in the Ukrainian Danube delta (Odonata-Anisoptera: Aeschnidae, libellulidae). Entomol Bericht 68, 188–190 (2008).