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).

Migrant hawkers scramble competition

I'm used to watching migrant hawkers foraging over gardens, leafy streets and sheltered woodland rides, some times in groups. They are immature individuals, gathering energy away from water. Migrant Hawkers, unlike other hawkers, mature slowly, and will move to suitable breeding sites after their long immature period. During this past week I've watched them in their breeding sites in lakes and drains, where mating and egg-laying takes place.

A mature male rests briefly between bouts of searching (Pickering Park, 27th August) 

Males at the breeding site 
At one of my local parks (Pickering Park) last week, dozens of Migrant Hawkers sat or patrolled alongside marginal vegetation around the lake. The males, now fully mature and showing their bright blue spots and eyes and side yellow-green stripes often hovered in a spot, or explored the vegetation, flying well into it, searching for females.

A typical hovering male in a clearing at the marginal vegetation (Pickering Park, 27th August), offering them good views.

Mating
I saw no females, until one was captured by a male: no preliminary or courtship, the male just tackled her and positioned himself to grab her by the head. The female is then able to curve her abdomen and mate, retrieving sperm from the male's secondary genitalia at the base of the abdomen, forming the 'wheel position'. They may fly in wheel position very fast, zigzagging alongside the marginal vegetation edge or briefly rising into the air, before settling on vegetation (above).

Ovipositing female (Foredyke Stream, 1st September)

The moment when the male passes by, and sees the female.

The pair, mating.

Oviposition
A couple of days ago I watched a patrolling male on a ditch doing its usual patrolling routine, rising to inspect any passing individual, even paying attention - briefly rising - to birds flying over. A mated female arrived, unnoticed, and started laying eggs on live leaves well above the water line, I'd say over one metre over the water. She checked leaves and unsheathing her ovipositor, started laying into them (the eggs will overwinter inside the plant leaves, where they are protected from predation). After a couple of minutes, the male noticed her, tackled her and mating ensued. This time the pair settled briefly on plants, which allowed me to take a shot (above). Mating in Migrant Hawkers is longer than in other territorial relatives.

Two males resting near each other (Pickering Park, 27th August)

Nonterritorial males
This species is notoriously non-aggressive, even at the breeding sites. Males will even rest within view from each other (above). A patrolling male will swiftly rise to check a passing one, but the interaction is suggestive of them 'checking' that they are not a female, and letting the other individual go their way if it's a male. There is no defended territory, just males congregating on suitable ovipositing sites and searching for females, a type of mating tactic called 'scramble competition'. This appears to be the reason behind the long copulation. A territorial male mating for a long time may lose the territory to an invader, or miss extra mating opportunities. A nonterritorial male has less to lose, and therefore makes sure that he fertilises as many as possible of the females' eggs, possibly by taking time to remove any previous sperm before transferring his transfer. These patterns have been shown to stand when the copulation duration of territorial and nonterritorial drogonfly species were compared, but I haven't found specific data on the Migrant Hawker. To illustrate the pattern, the Emperor, a territorial species, copulates for an average of 10 min, while the Common Hawker, a non-territorial one, copulates for an average of 67 min. Of course, this lengthy copulation is not necessarily to the benefit of the female, who may be already mated, as the female already laying eggs, and therefore this sets the stage for the evolution of female counter-tactics, such as visiting the water as little as possible and avoiding males if they can, something I have covered before at Bugblog.

More information
Córdoba-Aguilar, A., Serrano-Meneses, M. A. and Cordero-Rivera, A. Copulation Duration in Nonterritorial Odonate Species Lasts Longer than in Territorial Species. Ann. Entomol. Soc. Am. 102, 694–701 (2009).

The Migrant Hawker unusual life history

Migrant Hawkers have been around since the beginning of August, and they should be around until October or early November. They are often seen flying away from water. They hawk at about 3 m high, patrolling an area repeatedly, occasionally rising to catch an insect, their abdomen held at an upward angle, with a slight droop at the end. 

Migrant Hawker in flight.

Their late summer appearance and social, non-aggressive behaviour are unlike other hawker dragonflies in the UK. When they hunt, it is not unusual to see several individuals together, either ignoring each other or not overtly aggressive. Occasionally they form large hunting aggregations in favoured habitats. They also often rest near each other in pairs or small groups in sheltered glades or hedgerows after hunting or during overcast conditions, hanging from branches orienting to the sun (top shot, a female in the foreground, with a male on the background). 

The reasons behind their distinctive life history lie in their ecology. Migrant Hawkers have a widespread distribution, from the Mediterranean to Asia to Japan. In the south of their range they often develop in temporary ponds, which recurrently dry in the summer. For this reason they have a very fast life cycle: after laying in the autumn, eggs undergo a period of winter rest (diapause). Larvae hatch in late winter and develop through spring, before metamorphosing and emerging as adults in late summer. The whole cycle lasts a single year, unlike other dragonflies that take at least two years to develop. When the adults emerge, the ponds where they developed quickly dry, so the adults do not mature immediately. They will hunt away from water for a while - up to 4 months in the south of their range in Algeria - and only return to ponds and mature when autumn rains may have fill the ponds, therefore they have one of the longest adult lifespans in dragonflies. During their immature stage they are great wanderers and migrate in both latitude and altitude to new areas where new ponds can be found or weather conditions more benign. In the summer, when they are away from water, immature migrant hawkers have no reason to be aggressive to others, there is no territorial behaviour or competition for receptive females, hence the congregations in feeding swarms on the plentiful insects. 

Male Migrant Hawker.

 Migrant Hawkers used to be rare in the UK before the 1940s, but they are now widespread in England and Wales, and also found in Scotland. In the early 2000 they colonised Ireland together with two other dragonfly species. It is likely that increasing temperatures are allowing this species to complete its cycle in the UK, where previously it was just a migrant species and their range has spread north as most other British dragonflies.

More information
Hickling, Rachael, et al. A northward shift of range margins in British Odonata. Global Change Biology 11.3 (2005): 502-506.

Samraoui, B., Bouzid, S., Boulahbal, R., & Corbet, P. S. (1998). Postponed reproductive maturation in upland refuges maintains life-cycle continuity during the hot, dry season in Algerian dragonflies (Anisoptera). International Journal of Odonatology, 1(2), 119-135.

Three bright characters for the end of August

Some invertebrates appear as heralds of autumn, and today three striking ones were present in the garden. Migrant Hawkers have been around for a while, forming loose swarms that hunt between 2 and 8 m high, often well away from water. In the cooler, shorter days of late summer it becomes easier to come across sunbathing ones, perched on a branch, often more than one near each other. I flushed this male a few times as I went about in the garden, until I finally spotted him hanging from its perch. I got so close taking the macro above that I could have kissed him.

A single Red Admiral was also about, alternating between feeding in the buddleia and basking on a brick wall. During sunny spells she closed its wings, while during passing clouds she revealed its fresh, amazingly marked wings to their full splendor. This was a beautifully marked individual, the small, delicate blue markings on the edge of its wings very apparent.

I used the flash to counteract the sunshine and reveal the intricate patterning of the underwings, which can make the butterfly well camouflaged.

This garden spider, Araneus diadematus, is one of the largest in the garden, she hangs her web on the side of the rubbish bin, and has her retreat under the rim. Given the size of this species, detailed inspection of the epigyne is possible without even disturbing the spider (click on the photo to see it).

Insects in flight

Today it was a summery, hot day, with a light breeze, ideal for insect watching. Somehow, I managed a few decent in flight shots, which usually evade me as I am not patient enough. The first one happened when I spotted two hovering Volucella pellucens. They are large hoverflies, which live near trees and are identifiable even at several meters of distance due to their translucent abdominal belt. Males hover incessantly on the same spot, and inspect or chase off other hoverflies and even speckled woods invading their territory, so maybe this way they spot female hoverflies to mate. Hoverflies also hover during courtship. I tried to encourage them to sit on my finger by slowly raising it towards the hoverfly. This technique often works with hovering males, like this Eristalis intricarius from yesterday shows:

but the Volucella refused several times...

So I tried to get some shots while it hovered and got the shot at the top of the post, and I was pretty pleased with that!
 A bit later, then, in my street, a couple of migrant hawkers were hunting over the verges. Migrant hawkers often hunt together with other individuals, and they may settle to bask near each other. They tend to hunt from 4-5 m above ground. I managed some records shots, this one the best.

I popped in the wildlife garden later to release a grasshopper (a matter for a different post) and watched a large Ectemnius digger wasp (probably the common E. cavifrons) inspecting a rotten log. She had to fend off a Tegenaria spider. Ectemnius are hoverfly hunters and are skilful hovers. Females dig their nests in soft wood, and look by suitable nest sites, often hovering in front of the wood. This allowed me an opportunity for the third flight shot for the day.

The Migrant Hawker

The Migrant Hawker, Aeshna mixta, is one of the commonest dragonflies found away from water. It regularly visits my pond-less garden, with a clearly marked peak in the second half of August (the male on the top shot from 15th August this year). This species is a fast developer, their larvae take just a year to become adults, but it is sensitive to the cold. Adults are not territorial and are often found hunting together, at high, patrolling over the garden hawking for insects. Early in the morning they rest hanging from vegetation, sunbathing, at about 1-2 m high, often a pair not far from one another, and they then allow close approach.

A female Migrant Hawker basking, with another individual on the background (16/08/06) - which I only noticed when checking the photos.

Although dragonfly migration has been known for over a century, it is still a little known phenomenon. Several British dragonfly species are regular long-distance migrants, like the Four Spotted Chaser and the Common and Ruddy Darters, and others occasionally appear as vagrants from the Mediterranean or even from North America. Where do they go and why do they do it? Do they migrate back and forth or do they just wander? Is this behaviour to avoid overcrowding or drought, in search for good sites for reproduction? David and Elizabeth Lack (1951) favoured the existence of large scale two way migration in dragonflies, like that of many birds and butterflies, after observing large numbers of Common Darters passing over a high mountain pass in the Pyrenees. American migrants tend to be species breeding in ephemeral or semipermanent ponds, which dry frequently, so migration might also be a wandering in search of suitable ponds. Recently, a combination of miniaturised radio transmitters, isotope analysis and genetic markers have shed light into the migratory behaviour of the American Common Green Darner, Anax junius. Although the Migrant Hawker has been seen forming large swarms, unfortunately, we know very little about the migration in this species. In any case, its migratory behaviour makes it a very good coloniser. In the early 20th century it was regarded as a rare immigrant - in fact it was called Scarce Hawker -, but since an invasion in 1935 it has been steadily and rapidly expanding its range northwards and westwards in the UK, reaching Scotland in 2004. The species also colonised Ireland in 2000 and is now well established. I can't wait to find out more about these fascinating garden visitors.

References
Wikelski, M., Moskowitz, D., Adelman, J., Cochran, J., Wilcove, D., and May, M. (2006). Simple rules guide dragonfly migration Biology Letters, 2 (3), 325-329 DOI: 10.1098/rsbl.2006.0487

Lack, D. and Lack, E. (1951). Migration of Insects and Birds Through a Pyrenean Pass The Journal of Animal Ecology, 20 (1) DOI: 10.2307/1644

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

Brian Nelson, Colm Ronayne and Robert Thompson (2003). Colonization and Changing Status of Four Odonata Species, Anax imperator, Anax parthenope, Aeshna mixta and Sympetrum fonscolombii, in Ireland 2000-2002 The Irish Naturalists' Journal, 27 (7), 266-272

September bugs

There is a definite autumnal feel these days. Shorter days, ripening berries and leaves already changing colour. September, however, is a month plentiful on all sorts of bugs. During a walk in our local Millennium Wood, Oppy Wood today, we spotted five butterfly species, Comma, Speckled Wood, Small Tortoiseshell, Red Admiral and Large White. We watched Commas and Speckled Woods sucking the sweet juice of ripe Blackberries.

There were plenty of Daddy Long Legs (tipulidae). Some of them mating.

A couple of Migrant Hawkers patrolled above our heads. A male (top) eventually stopped to rest, hanging from some low branches. He let me get really close to him for some shots.

There also were grasshoppers about - possibly Lesser Marsh Grasshopper, Chorthipphus albomarginatus - 7 spot ladybirds, froghoppers and wolf spiders.

A male Lesser Marsh Grasshopper

The Hawkers

For a week I have been watching hawker dragonflies hunting over the garden well into the evening. They fly high (3-5 m) and never seem to stop, so I could not identify them. My luck changed today, when early in the morning I came across a male Migrant Hawker (Aeshna mixta) sunbathing on a fence outside. This is the only species of hawker I have identified in my garden and I usually see it in August and September. It is quite approachable and getting close-ups is relatively easy.

The male was a little later joined by another individual (a duller male I think) and both sunbathed quite close to each other high up in a buddleia for a while.

My usual basic photographic equipment was joined by a chair, so that I could take the pictures. At 11:30 both dragonflies started hunting and they didn't stop again.

This species of dragonfly is very sociable and regularly hunts in groups. Is also, as suggested by its name, a regular migrant species. Before the 1950' it was quite a rare species in the south of the UK but steadily increased its range and is now found also in S. Scotland and Ireland.