Don’t get the blues from the Blues: a guide to identifying blue species of butterflies.

How can you tell apart the different types of Blue butterfly species? For some, this is the million-dollar question that can cost hours in agonising confusion. So we have put together this guide to save the day and help you see the subtle differences which will enable you to identify these beautiful blues with ease. As two of our beautiful blues are included in the Big Butterfly Count, the Holly Blue, and the Common Blue, we explore their differences below. In most parts of the UK and in most habitats, these are the only two butterflies that you are likely to see. But be aware that if you are on chalk or limestone grasslands, lowland heath, sand dunes or a few other special habitats, there are other blue butterflies that can cause confusion and are not dealt with in this blog. You can see the other blue butterflies here:
Adonis Blue
Chalk Hill Blue
Large Blue
Silver-studded Blue
Small Blue

The Common Blue is (not so shockingly) quite common and can be found far and wide across the UK. It is a grassland butterfly, usually found where there is uncut grass and wildflowers. These butterflies usually fly close to the ground, regularly visit flowers to feed and (you might be surprised to learn) are uncommon in gardens, particularly in urban areas. 

The Holly Blue, in contrast, is very much at home in urban gardens, parks and cemeteries. It is also commonly seen in woodland and along hedgerows. They are a similar size to the Common Blue, but Holly Blues usually fly higher, often around trees and shrubs rather than staying close to the ground. It is also less common to see Holly Blue butterflies on flowers, although they do visit them from time to time.

So, you can already make some assumptions based on where you have seen a blue butterfly and what it was doing. Garden sightings are much more likely to be Holly Blue, especially if the butterfly is flying around trees or shrubs.

A good look at the butterfly is essential, however, to confirm identification. Both male and female Holly Blues and male Common Blues have blue upperwings and look blue in flight. Female Common Blue butterflies vary from brown to mostly blue, but always have a distinctive series of orange spots towards the edges of their wings, that Holly Blues don’t have.

The key distinguishing feature is the undersides of the hindwings. In both sexes of Holly Blue, these are pale blue with numerous small black dots and no other markings. In contrast, Common Blues have grey or brownish undersides with complex markings including a row of orange spots on each wing and rows of white spots with black centres.

Holly Blue Male - Allestree 15th April 22 - Ken Orpe.JPG
Male Holly Blue, Ken Orpe
Holly Blue (male/underwing) - Iain Leach
Holly Blue, male

If you can’t see the underside of the wings, you can still tell Holly Blues from male Common Blues on the basis of their upperwings. Common Blue has lilac-blue coloured upperwings with a clear white outer fringe. The Holly Blue has dark lines running across the white outer fringe, giving a more chequered appearance. In addition, the female Holly Blue has a thick black marking towards the edge of the wing, which is not present on Common Blue.

Common Blue Male - HGQ - Dave Gilbert.jpg
Male Common Blue, Dave Gilbert
Common Blue [m], Dawlish Warren NNR, 24.5.22 (Dave Holloway)
Male Common Blue, Dave Holloway

To find your ID checklist please visit the Big Butterfly Count website to download the guide.