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).

Urban bush-crickets: Roesel's Bush-cricket

I'm starting a daily series on urban bush-crickets. Checking through the blog, I realised I haven't paid much attention to these newly arrived species in the city. In 2022, I undertook a survey of urban bush-crickets, after I was made aware of the presence of several species in the city of Hull. It was an exhilarating summer: armed with a heterodyne bat detector, I explored grassy areas, marshland and brownfield sites across the city, listening for them or trying to spot them. The last two summers, I have continued with the surveys, as I have really got the bush-cricket bug! With this series, I want to write what I wish I had known when I started regarding each of the species.

Female Roesel's Bush-cricket basking, Rockford Fields, 9 October 2023. Note the curved, relatively short ovipositor and short wings.

The Roesel's Bush-cricket, Roeseliana roeselii, is a stocky species (top shot, a calling male at Sculcoates Cemetery, 28 September 2024). On first impression, if you see one scuttling through grass you think it's a small toad, as they physically disturb the vegetation with their bulk as they move. The size and colour is variable. The most common form is brown and yellow, with some green touches, but some individuals can be very green. The rim of the black side of the saddle-shaped thorax is white, yellow or green. Three yellow spots just above the base of the legs, on the thorax, are distinctive, as is a black comb-shaped stripe across the jumping legs. The species is wing-polymorphic. Nymphs growing in dense populations tend to develop long wings and are able to fly long distances. The long-winged form is responsible for long-distance dispersal and colonisation of new areas. The most common form has short wings, extending half way through the abdomen, and is unable to fly. Females are larger than males and have a short ovipositor, curved upwards. Males have modified forewings, which they use to sing (stridulate). They lay their eggs in plant stems. During mating, males provide females with a nuptial gift called 'spermatophylax' which makes a substantial proportion of males' weight. 

A male long-winged Roesel's form calling.

Behaviour. Individuals like to bask in sunshine, often quite exposed, especially in cool mornings in the autumn. Sometimes they will stretch out a jumping leg to expose the abdomen to the sun. Males start to sing while basking, choosing grass tussocks as a perch. Tussocks offer individuals protection from predators: at the slightest disturbance, they drop onto the base of the tussock and scramble into it. They are not particularly social and will compete for a good perch or sunny spot.

Roesel's are very well camouflaged in their environment and can be tricky to spot. Try to find a calling male Roesel's male from a perch on tall grass. (tip: the Roesel's is at the bottom third of the centre of the image. There is also a Long-winged Conehead on the centre top third.

A typical urban unmown field holding Roesel's.

Habitat. Thought to be associated to salt-marshes and dunes, the species has been shown to be little limited by habitat and can now be found along road and path verges, grassy fields, brownfield sites, and any habitat that includes rank grasses. 

A Nymph Roesel's busch cricket. 28th May 2023, my first observation of the year for the species.

Field craft. How do you find Roesel's bush-crickets? The first nymphs appear in mid to late May. Nymphs can be found by sweeping grass with a net. Adults appear at the end of July, and are best searched for. Male's stridulation is within the human audible range, but it is a high-pitcher whirr, that is easier to hear for children. Many middle age people are unable to hear them at all, so using a bat detector set to 20 KHz makes it easier to detect them. The call is a continuous whirr that has been compared to an electric buzz. Once you are close enough so that the sound is quite loud, you can try and pinpoint where the calling male is perched by swaying the bat detector left and right and up and down. I find binoculars very useful, to scan suitable perches from a couple of meters with minimal disturbance to spot individuals. With a bit of luck, once located, using slow movements, they can be approached to close range. Males tend to call during the day in sunny weather, unless they are already warm, then they will carry on calling. Searching at suitable sunny spots for basking individuals can also be rewarding, especially on bramble leaves next to grass, where they can be more obvious. Roesel's season winds down as October progresses and the last adults die off with the first frosts, their eggs surviving in grass stems over the winter. 

A long-winged female Roesel's. I saw it flying and landing on the short grass of a playing field (31st July 2020), this is the only photo I took, as it immediately flew north. 

Range expansion. Roesel's have undergone a spectacular range expansion in the last decades, from the south east corner of the UK. The first record in Hull was in summer 2015. They are now widespread in East Yorkshire and the northernmost populations are currently in North Yorkshire (at Robin Hood's Bay), but the colonisation is ongoing, aided by climate change and by long-winged forms that appear once population density is high.

A mating pair of Roesel's Bush-crickets. The female is much larger and above, whilst the male hangs underneath the female.

A female eating the spermatophyllax.