These belong to the same family as the "Blues" (Lycaenidae).
They nearly all have small "tails" on their hind wing (except
the green hairstreak).
Some of the blues outside the UK share this character (see
Long-tailed Blue) so it is not distinguishing, but it is
good enough for the UK observer.
They are generally hard to find due to either their habit
of preferring tree tops (Purple Hairstreak), or their scarcity
White Letter Hairstreak 04
Green Hairstreak 1 (S)
Black Hairstreak 06
A beautiful butterfly that is very scarce. They require
large tracts of Blackthorn to breed. Photographed
at Bernwood Meadow Wildlife Trust site near Oxford.