Sunday, 3 September 2017

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 between their distinctive life history lies 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.

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