Common Blue Butterfly on the Sefton Dunes: June Is the Month

28 May 2026

Why the Dunes Work for Common Blues

The common blue butterfly (Polyommatus icarus) is a species of short-grass habitats with abundant legumes. Its larval foodplant is bird's foot trefoil and, secondarily, kidney vetch. Both plants grow in good quantities in the Sefton dune slacks and on the dune slopes where turf is short and open.

The dune system provides the warm, sheltered, south-facing slopes that common blues require for territory establishment and basking. The mosaic of open sandy areas, short turf and flowering legumes creates ideal habitat across a large continuous area at Ainsdale and Birkdale Hills.

This is not a rare butterfly nationally, but the quality of the Sefton dune population is notable. On a good June day with warm sunshine, you can see dozens of individuals in a relatively small area of suitable habitat.

Males and Females

The male common blue is electric blue on the upperwing with a white fringe. In good light, the colour is vivid enough to be visible at 30 or 40 metres. Males are territorial and will perch on prominent vegetation stems, periodically chasing rivals and returning to the same perch.

The female is mostly brown above with orange spots along the wing margin and a variable amount of blue at the wing base. It is easy to overlook females as they blend into vegetation, but they are present in similar numbers to males and spend more time close to the larval foodplants.

Underwings in both sexes are pale grey-brown with orange spots and white-ringed black dots. The pattern is distinctive and useful for identification when the butterfly is settled with wings closed.

When and Where to Look

Peak flight period is June, with a second generation in late July and August. The first generation is typically larger and more numerous. June sees both sexes active simultaneously, making it the most productive time for observation.

Best conditions: warm (above 17C), sunny, low wind. Common blues are sensitive to temperature and largely inactive in cloud or below 15C.

The dune slacks at Ainsdale NNR are the most reliable location. Walk the boardwalk paths and check the open areas of short vegetation around the slack margins. The slopes of the dune ridges facing south and southwest also hold territories.

The Birkdale Hills LNR is a quieter alternative with similar habitat and less visitor pressure. Access from Shore Road or the coastal path from Southport.

Associated Species

Where you find common blues, look also for small heath, which is abundant in the same habitat. The small heath is a small orange-brown butterfly that always settles with wings closed, showing a distinctive eyespot on the forewing.

Six-spot burnet moth shares the dune grassland with common blues in June and can be seen nectaring on the same plants. Its red and black colouring makes it unmistakable.

Brown argus is a smaller brown butterfly with orange spots, often confused with female common blue. It is less common on the Sefton Coast but present. The best way to separate them in the field is by the absence of any blue scaling on the upperwing in brown argus.

Conservation Context

The common blue has declined across much of lowland Britain as rough grassland and downland has been lost to development, improvement and scrub encroachment. The Sefton dune system has remained viable partly because of active management: dune slack restoration, removal of invasive sea buckthorn, and grazing management that maintains the short-turf conditions the butterfly requires.

The work of Natural England at Ainsdale NNR and the ongoing management of Birkdale Hills directly benefits the common blue population. Any visit to these sites supports the case for continued investment in dune management.