Thursday, 12 August 2021

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:

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).

Saturday, 31 October 2020

Budapest slug mating

It's the last day of October: damp, windy and mild. As I go out into the garden, pondering upon the impending winter lockdown, I find two Budapest slugs circling on a cherry leaf on the ground. Budapest slugs remind me of hedgehog poo, dark and shiny and all the right shape. If they had been on the path pebbles I would have easily missed them. I watch them for a few minutes. They keep slowly circling, head-to-tail for a while as they follow each other's mucus trails. This is slug courtship and it happens at slug pace. It's 10:40 and it doesn't look like much is happening any time soon. I regularly go out to check on them for the rest of morning and afternoon. As I'm typing this, I grab a torch and decide to check on them again. They are still there, exactly in the same position as 3 hours ago. 

10:42
11:06. The pair have moved onto the path pebbles now, the circling has stopped. They are now finding each others genital openings at the right side of their heads.
11:20. Copulation proper appears to start.
12:20
14:31
17:20. Very little change in the last few hours. The slugs are now entwined and practically immobile.
I pick the Slugs of Britain and Ireland and check the species account for info on their mating behaviour. Of note is that they are mainly subterranean and active year round.
This quote is fitting: 
"in winter adults are often found almost motionless, mating. Mating lasts many hours and involves the production of elaborate spermatophores"
I wonder if they'll be there tomorrow.

More information
Rowson, Ben, James Turner, Roy Andreson and Bill Symondson Slugs of Britain and Ireland. 2014. FSC Publications. AIDGAP.

Tuesday, 27 October 2020

Emperor home ranges, habitat use and differences between sexes

Emperors (Anax imperator) have done very well this year in my local area, which has allowed me to become familiar with the different behaviours of males and females. Females are quite stealthy, entering ponds to oviposit on floating vegetation, often flying low over the water. In contrast, males display an obvious territorial behaviour, patrolling high over the water of the large pond, lake or slow running drains where they breed. Males move towards passing birds, and towards any intruders, with spectacular fights and clashes between the males, some times resulting on individuals falling on the water.

Ovipositing female emperor.
A male patrolling alongside the marginal vegetation of a large ditch.

All this means that we are biased in our understanding to these more obvious behaviours, ovipositing and territoriality. But many other questions remain about Emperor's behaviour and ecology, for example, where are females when they are not ovipositing? or males, when they are not patrolling a pond? how far do individuals roam or disperse? where do they roost? how is their behaviour affected by temperature? do they make use of the habitats around ponds? what about sex differences?

Capture-mark-resighting techniques have be used in dragonflies to study individual movements, behaviour or demographic parameters. Individuals of large dragonflies can be marked using alpha-numeric unique wing codes written with permanent markers, which could be read from a distance, with no need from physical recapture (for example using binoculars). This technique, however, requires an enormous field effort to relocate as many individuals as possible.

Another technique that has been used to study migration is to analyse location-specific isotopes in wing samples. These isotopes have signatures specific to geographic areas, which point at the area where the larval stage took place (as the wing tissue is formed during the larval stage). This method has been used to reveal the multi-generational migrations of the Green Darner (Anax junius) a North American relative of the Emperor that is a long-distance migrant. Hydrogen stable isotopes showed that the migration cycle comprises a north migrating generation, a south migrating generation and a resident generation that develops around the Gulf of Mexico. Another study using stable isotopes on the Global Skimmer, Pantala flavescens, revealed its multigenerational migration steps around the Indian Ocean. Although ranging from South Africa to Sweden, and still involved in natural colonisation towards the north, facilitated by global heating, the Emperor is a resident species, so this technique is not of much use.

Radio transmitters in dragonflies?

As technologies have resulted in the miniaturisation of radio-transmitters  in recent years, they have increasingly been used to study more local movement patterns in large dragonflies, like the North American Tiger Spiketail (Cordulegaster erronea) a relative of our Golden-ringed Dragonfly and Green Darners. The individuals need to be captured and fitted with tiny transmitter before release, and they have to be found in the landscape using scanning receivers fitted with an antenna. Size matters as the transmitter must be less than 30% of the weight of the dragonfly not to impede normal behaviour. The Emperor, one of our largest dragonflies, weighs about 1 g, and can be fitted with such small transmitters (check the photo here of an individual fitted with a transmitter). In an early small scale study (5 tagged individuals), researchers looked at home ranges and local movements between roosting sites and pond territories in male Emperors. The furthest moving male travelled 1.5 km from the tagging pond to another pond.

An article published recently sheds some light on home ranges and habitat use of Emperors using radio-transmitters. Marceau Minot and his collaborators chose five ponds in an urban/rural interface in northern France. Over the summers of 2017 and 2018 they marked 87 mature emperors with unique wing codes and visited the ponds at least once weekly to search for marked individuals. They also fitted 54 individuals with radio transmitters, and tracked them daily for up to 15 days.

Capturing individuals to fit radio transmitters or mark their wings has a cost. Both techniques increase mortality in the day after capture, probably due to the stress of the capture. 

Sex differences in behaviour

Females had larger home ranges than males. The furthest distance a female travelled was 1.9 km while males moved less, with the maximum male movement 0.5 km. This could be related to male's territorial behaviour. Presumably females oviposit in several ponds.

Reproductive behaviour of males, but not females, is positively related to temperature.

Flying behaviour in females is positively related to temperature, while resting high in trees is negatively related to temperature.

Both sexes were mostly found on ponds or pond marginal areas, although males spend more time near water.

Resting happened in ponds and trees. Females tend to roost high on trees, more than males. While males tend to rest low in vegetation.

A male emperor resting on marginal vegetation.

The researchers estimated the effect of marking protocol on survival of the dragonflies. The manipulation of the individuals affected their survival, possibly due to the stress during manipulation: 76% of individuals survived 24 after capture and wing marking, while just 56% survived 24h after being fitted with a radio-transmitter. Individuals with proportionally larger wings (not larger body mass) and younger in age survived better throughout the experiment.

Habitat management

The study also highlighted that both rural and urban ponds will benefit from the presence of neighbouring trees as suitable roosting sites, and emperors will benefit from the presence of a network of ponds, rather than isolated ponds.

More information

Hallworth, M. T., Marra, P. P., McFarland, K. P., Zahendra, S. & Studds, C. E. Tracking dragons: stable isotopes reveal the annual cycle of a long-distance migratory insect. Biol. Lett. (2018).

Hobson, K. A., Anderson, R. C., Soto, D. X. & Wassenaar, L. I. Isotopic evidence that dragonflies (Pantala flavescens) migrating through the Maldives come from the northern Indian subcontinent. PLoS One 7, e52594 (2012).

Knight, S. M., Pitman, G. M., Flockhart, D. T. T. & Norris, D. R. Radio-tracking reveals how wind and temperature influence the pace of daytime insect migrationBiol. Lett. 15, 20190327 (2019).

Levett, S. & Walls, S. Tracking the elusive life of the Emperor Dragonfly Anax imperator Leach27, 59–68 (2011).

Minot, M., Besnard, A. & Husté, A. Habitat use and movements of a large dragonfly (Odonata: Anax imperator) in a pond network. Freshw. Biol. 46, 207 (2020)

Wednesday, 30 September 2020

Hull Dragons 2020: September summary

Autumn is definitely on the air, after the warm and sunny days that have brought the dragonflies and damselflies out in the first three weeks of the month, cooler weather with northerly winds and more rain set in for the last part of the month. A total of 133 records from 8 species were submitted in September by 15 observers, covering 31 km2. Thank you all for your efforts. The survey is set to surpass the number of records submitted last year, with a total so far of 696 records. 

March of the Willow Emerald
The Willow Emerald Damselfly has expanded through the UK since 2009, after natural colonisation from Europe. A year ago Dick Shillaker found a male Willow Emerald at East Park, the first record for East Yorkshire. This year, we've had a number of records for the species in the Hull area, in fact, it has been the 3rd species in number of records for September after Migrant Hawker and Common Darter. A warm, breezy afternoon - one year to the day of the first record - I bumped into a mating pair, first flying and then settled on a buddleia branch, just under the pedestrian bridge at Abbey Way, over the Beverley & Barmston drain (top shot). Some distance away, a lone female basked on a Japanese Knotweed. A few days later, we had another set of records at a new location, Humber Bridge Country Park, where 6-7 individuals with two mating pairs in the same area and records for a number of days. A male was spotted at the Beverley and Barmston Drain by Beresford Avenue. The sudden increase in records from last year and their spread, and the number of individuals involved may suggest that the species may have been overlooked last year, when it might already have bred in Hull. Alternatively, there has been a massive influx of individuals arriving at suitable sites coinciding with the warm weather and southernly winds?
Female Willow Emerald Damselfly on Japanese Knotweed.

Beverley and Barmston Drain
I have been visiting the Beverley and Barmston drain regularly this year. It has been surprisingly good for dragonflies, with even a Hairy Dragonfly early in the year. The drain does looks like ideal habitat for Willow Emeralds: very slow flowing and with occasional willows over the water. 
A terrapin emerges from the blanket of duckweed
Moorhen sat on floating rocking horse.
A stretch of drain with overhanging willows.
This stretch of the drain has large trees on one side.

Abbey Way pedestrian bridge.
Beresford Avenue bridge.
Breeding Migrant Hawkers
September is peak breeding season for Migrant Hawkers. They reach maturity and males develop intense blue and yellow colours. Females also have more rich brown and yellow colours. Individuals may still be seen away from water, but as they mature, they are increasingly found by their breeding lakes and ponds where their behaviour becomes more focused on breeding. Males patrol areas by marginal vegetation, often hovering over this for some seconds before moving to prospect another area. Sometimes they enter marginal vegetation searching for females. If a female is spotted, mating ensues. I have been able to follow two mating pairs from the beginning. The pair flies away from the water and settles on trees or marginal vegetation, on a south facing spot. Although I've watched many pairs mating, in two instances I could watch a mating pair for a long time, since the initial capture of the female by the male. These two pairs remained mating for over 32 minutes and over 17 minutes (I didn't stay the full length of the mating so these will be underestimates). I have been unable to find any information on the average mating length in the species. In a previous post I have covered the mating system in this species, which concludes that non-territorial species, like the Migrant Hawker, where sperm competition is likely to be more intense, mate for longer, and is nice to see that my observations match this expectation.
A resting male in full breeding colours.
Mating pair at Noddle Hill lake.
Migrant Hawker ovipositing on floating branch at Foredyke Green Pond, an unusual ovipositing spot for this species.
Female Migrant Hawker ovipositing on branched burr-weed leaves, by the water. 
Brown Hawker ovipositing on floating wooden board.

Species recorded in September

Migrant Hawker, 74 records.

Common Darter, 30 records.

Willow Emerald Damselfly, 11 records.

Southern Hawker, 6 records.

Small Red-eyed Damselfly, 2 records.

Blue-tailed Damselfly, 1 record.

Ruddy Darter, 1 record.

Brown Hawker, 1 record.

Wednesday, 2 September 2020

Hull Dragons 2020: August summary

We've had some excellent dragonfly weather this August, with plenty of warm, sunny days. Although the last week had plenty of wind and rain, it was often warm enough even when overcast to entice some dragonflies out, until the very end of the month, dominated by cold northerly winds.

Odonata highlights

We've got a whooping 200 records from 12 species in the month of August. These are from at 35 1 km squares for August from 13 recorders, thank you all who have contributed records!

The season is over for many species (Hairy Dragonfly, Large Red Damselfly, Broad-bodied Chaser, Banded Demoiselle), and is coming to an end for others: Azure Damselfly, Four-spotted Chaser or Black-tailed Skimmer.

Migrant Hawkers (top shot) have been the top species, with a third of all, followed by Common Darter,and Blue-tailed Damselfly. Although there has been no new species added for the year in August, there is a chance we may still record Willow Emerald as the first records las year were in September.

Brown Hawkers

Brown Hawkers bumper year has continued, they are the 5th species in number of records for the month. In July many observations involved patrolling males on territory, but in August, ovipositing has been observed, twice at Foredyke Green (on the muddy shore, on floating wood and on floating polystyrene) and also likely ovipositing behaviour at East Park.
Female Brown Hawker rests between ovipositing bouts.
Oviposition on polystyrene.

Migrant Hawker

Most records involving hawkers away from water, hunting at 4-5 m high, sometimes in twos and threes are highly likely to be Migrant Hawkers in our area. Two or three individuals have been present regularly in my garden, and I was lucky enough to spot one of them basking quite low on a potted olive tree. It allowed for very close approximation.
A male Migrant Hawker Photo taken by my son with his mobile phone. 22nd August.
And a female, from the 31st of August.
There were several exciting Migrant Hawker highlights. One of them was to find a total of 6 exuviae at East Park. Exuviae is the formal name for a moulted larval skin, which can be found on marginal or emergent plants, after the adult has emerged. The presence of exuviae in a site provides evidence that a species has successfully bred in a site. As far as I know this is the first confirmation of successful breeding in the city of Hull after last years observed ovipositing in several sites.
Migrant hawker exuviae in situ.
Another interesting observation involved an individual flying underneath an ivy overhang and settling on it, moments before a rain shower. I found another individual near it. I wonder what makes the hawkers seek refuge, maybe the sudden darkening of the sky?


I was at Pearson Park Wildlife garden during muggy, but cloudy weather. During a sunny spell followed by a strong breeze, at least 20 migrant hawkers took to the wing and started hunting over the garden. This is the largest number of individuals I have seen of this species.
Finally, I have been able to observe Migrant Hawkers active during cloudy weather and temperatures as low as 13-14 oC. They might be able to achieve temperatures high enough for flight by basking during brief sunny spells. When windy, they forage around trees at the lee side of the wind in sheltered, sun-facing spots.

Blue-tailed damselfly mating behaviour
The following photo show a mating pair of Blue-tailed damselflies. In this species, copulation is very long, up to 8 hours and in dense populations a form similar in colour to males increases in frequency, as these females avoid the costs of prolonged copulations.
Females oviposit alone. I was at Foredyke Green site about to take a photo of an ovipositing female, when a male came out of nowhere, knocked the female over, and immobilised her by holding her with jaws, legs and mating appendages. The following is a series of photos documenting how the male gains hold of the female, in the last photo he has already adopted the tandem position. I wonder had I only seen the last part of the behaviour, with the female already on the water, if I would have concluded that this was an example of a 'damsel in distress' being rescued by the male. I'm glad I got the whole sequence!

Small Red-eyed Damselfly

The Pickering Park and East Park populations are thriving: hundreds of individuals were present at East Park model boating lake and some as well in the main lake. A record in a new location was at St Andrews Quay Pond. The records show a establishment of the species in both large parks at Hull. Copulation and ovipositing was observed at East Park and Pickering park.

Small Red-eyed Damselfly at Pickering Park.
Small Red-eyed at East Park.
Small Red-eyed at East Park.
Copulating Small Red-eyed damselflies at East Park.

Best sites this year so far:

  • East Park: 13 sp.
  • Noddle Hill LNR: 12 sp.
  • Foredyke Green Pond 11 sp.
  • Oak Road Lake 9 sp.
  • Pickering Park 8 sp.
  • St Andrews Quay Pond 8 sp.
  • Beverley & Barmston Drain 7 sp
  • Paull Holme Strays 7 sp.
  • Humber Bridge Country Park 7 sp.
  • Pearson Park Wildlife Garden (closed due to COVID, but surveyed with permission) 6 sp.
  • Willerby Carr Dyke, 6 sp.
  • Beverley & Barmston drain, 6 sp.

Other species Gallery

Teneral Common Darter.
Common Darter in obelisk position
Emerald Damselfly at Noddle Hill LNR.
Ruddy Darter at Noddle Hill NR.
Southern Hawker at Noddle Hill LNR
Ovipositing Emperor at Foredyke Green pond.
Common Blue Damselfly.
A late Azure damselfly. Numbers have been steadily falling during the month of August.