Identifying features
Wingspan: 30-35 mm. The upperside of the male's wings has a blue sheen and is bordered by black edges.
Similar species
If one only looks at the underside, an Quercus can, from a considerable distance, be easily confused with the relative Brown Hairstreak (T. betulae).
Distribution
F. quercus is widespread throughout almost Denmark, but is absent in a "smaller" area in Northwest Jutland and is rare in the southern part of Zealand.
Biology
F. quercus is rarely observed, as the species typically inhabits oak canopies.