Our most common blue butterfly.
Where to see
- Habitat: Anywhere its caterpillar's food plant can be found: Downs, heaths, wasteland, old quarries, dunes, cliffs and any any rough unfertilised grassland that is fairly short.
- Caterpillar foodplants: Birds-foot refoil, Black Medick, Restharrows and (in boggy areas) Marsh Bird's-foot Trefoil.
- Best places: Cashmoor, Clubmens Down, Fontmell Down, Sovell Down, Ballard Down, Durlston Country Park, Townsend Quarry, Cerne Abbas, Portland Broadcroft, Portland Tout, Badbury Rings, Hod Hill.
Distribution map
Common Blue was recorded from these 1km squares in Dorset (2015-19).
- Key:
- 1 record
- 2-9 records
- 10+ records
When to see
The Common Blue has two broods, one peaking in June and the other in August to early September.
Sightings by month (last 5 years)*
This year and last*
What to look for
As with most blues, though the male is blue, the female is brown. It is a small butterfly compared to the Orange Tip, but middling size for a blue. It tends to fly low down.
The male is a lovely shade of plain blue, with a thin black band round the edge and an unbroken white fringe. There are often black marks round the wing edges but they don’t cross the white fringes. Unfortunately with age the white fringes tend to get worn away.
It is most easily confused with the Holly Blue, though a glimpse of the underwing shows these are very different – highly patterned with orange and black markings in the case of the Common Blue, and plain pale blue with small black marks for the Holly Blue.
You might also wonder if it is an Adonis Blue; the shades of blue tend to be different, with the Adonis being much more turquoise, but the best way to tell is to look at whether the black veins go through the white fringes: they do on the Adonis, but not the Common Blue.
The female is much less eye-catching and tends to stay hidden away a lot of the time. The ratio of brown to blue can be quite variable. Compared to the female Brown Argus, the female Common Blue has more blue scales on the surfaces of the upperside wings, and the orange dots on the upperside hind wings have black dots edged with white, while the Brown Argus has almost pure chocolate brown upper wings and the orange dots on the upper hindwings are edged by black dots.
Photo gallery
Click thumbnails to view full-size images.
Common Blue male, showing the white fringes and the small black marks around the wing edges, which do not cross the fringes, as do those of the Adonis Blue. Photo: Ken Dolbear.
Common Blues male. Note how the white fringes disappear with age. Photo: Brian Arnold
Common Blue female; all-brown upper wings. Photo: Ken Dolbear
Common Blue (female). Some blue but some brown: these three photos show how variable the female colouring can be. Photo: Keith Beswick
Common Blue; a very blue female. Photo: Gordon Cryer
Common Blue male, with grey background colour. Photo: Mark Pike
Common Blue female, with brown background colour. Photo: Andrew Cooper.
Comparison of undersides of Common Blue and Brown Argus. The BA lacks the spot on the underside of the forewing nearer the body than halfway that the BV has. Also, the double spots on the hindwing have different alignment: side by side on the Common Blue and one above the other on the Brown Argus . Photo: Robin George
ADONIS BLUE. Note how the black lines clearly cut through the white fringe. Photo: Peter Lister.
HOLLY BLUE. Note how different the underwing patterning is to the Common Blue.