Brown Hairstreaks begin to emerge at the beginning of August, when recently emerged males can sometimes be seen resting on the terminal leaves of ash saplings. Their first flight takes them to an assembly point - usually at the top of the tallest ash tree in the vicinity. In the absence of ash trees, sycamore, oak and other species are sometimes used.

Males spend most of their lives high up in these so called "master trees", where they feed on aphid secretions which coat the buds and leaves. They are rarely seen low down, but do occasionally descend to nectar at hemp agrimony or ragwort in years when aphid populations are low, resulting in a scarcity of honey-dew on the ash trees.

Brown Hairstreak Thecla betulae, female, Noar Hill, Hampshire

Freshly emerged virgin females can occasionally be found at rest on bushes in the early morning, but as soon as their wings are fully dried and hardened they fly up to join the males in the master trees, of which there are usually several serving each colony. Courtship is very brief and takes place in mid-morning on sunny days. Males have a patch of androconia ( scent scales ) on their upper forewings, from which they disseminate pheromones. As soon as the sexes meet the female responds to this chemical attractant by leading the male to a more sheltered spot lower down on the tree, where copulation takes place immediately. The pair remain joined for about an hour, after which the male returns to the tree tops, where he probably mates with further females.

The females are reputed to remain in the ash trees for about a week while their eggs are maturing, but thereafter spend their lives at lower levels. They are decidedly elusive, but with determination and luck can be discovered crawling about purposefully over young blackthorn twigs, probing with their abdomens to locate suitable places to lay their eggs. On a sunny morning a female will begin ovipositing at about 9.30am, laying perhaps 20 eggs in the course of the next 3 or 4 hours.

Between bouts of egg-laying the butterflies rest for long periods, usually on blackthorn bushes or ash saplings.

Brown Hairstreaks are generally regarded as being sedentary in behaviour, but my observations show that this is certainly not always the case. In late August 2010 for example, I visited Dunch Hill in Wiltshire where I saw 3 females, each on small isolated "islands" of blackthorn scrub, surrounded by broad stretches of grassland. I watched one female basking on blackthorn, and soon afterwards she took flight across an open expanse of grassland, eventually resettling on another blackthorn bush about 150 metres away. These isolated  bushes were about 1km distant from the nearest known site - a blackthorn thicket across the county border in Hampshire. Brown Hairstreaks were also reported at the same time from several previously unrecorded sites in Hampshire and Sussex, indicating that in favourable years females will disperse from their traditional habitats, and can turn up at least 2kms away from known sites.

Brown Hairstreak Thecla betulae, female, Tidworth ranges, Hampshire

Nectaring behaviour varies considerably from site to site - at Selborne for example both sexes are commonly seen at the flowers of hemp agrimony, but at Copsale they nectar almost exclusively at ragwort. Bramble flowers are visited at many sites, and less commonly fleabane or creeping thistle. Once they have settled to feed they often remain at the same clump of flowers for half an hour or more. Both sexes also commonly imbibe the sugary secretions which coat the black buds of ash; and can often be seen probing the surface of ash leaves, feeding on honey dew.

Both sexes feed with their wings tightly closed, but in weak hazy sunlight they can sometimes be found basking on blackthorn bushes or on low foliage, with wings fully outspread.