There are usually two broods in Yorkshire, with a small partial third brood in south Yorkshire.
Adults usually emerges from hibernation March to May though they may also emerge on occasional warm days throughout the winter. Butterfly lives for 10 months or more and some years they can be seen for at least a day or so during every month of the year.
After emerging from hibernation and finding a nectar source on which to feed, many Small Tortoiseshells use suitable weather to move on. They tend to feed in the early morning, and migrate in the middle of the day with males setting up a territory in the afternoon in the hope of intercepting a passing female. When a female arrives the male attempts to become her defender and will creep up behind her when she is at rest with wings open. He drums on her hindwings with his antennae and follows her about for the rest of the afternoon. If she is receptive she will lead the male to roost in a nettle bed, where the two will copulate all night on the underside of a leaf, unseen (and rarely photographed!). The next day may see the two butterflies move on (weather permitting). Specially marked individuals have been shown to migrate up to 150 kilometres, travelling short distances each day up to about four kilometres for each hour of sunshine available.
Green eggs are laid in clutches, up to about 100, on the undersides of nettle leaves. The female prefers nettles in full sunshine and often young plants on the edge of a clump. Several females may deposit eggs on the same leaves resulting in up to 1000 being found in a single cluster.
Eggs hatch after about two weeks and larvae build a communal home using silk threads to tie leaves together into an untidy bunch. Large clusters of larvae break up into smaller groups and each community will periodically move on to another plant and build a new communal home. Being gregarious they are able to benefit from the combined body warmth of the group and cope with cooler conditions better than many other species. They are also better adapted to surviving in warmer conditions, in which they can grow more rapidly. Young larvae are greenish-yellow with fine black hairs and develop through five instars into darker specimens with lateral yellow lines and dark spines, and are poisonous to many creatures, although still parasitised by ichneumon wasps and tachinid flies. When disturbed they can jerk heads in unison or regurgitate a ball of green fluid around their mouths.
ln the last instar they disperse up to 50 to 60 metres and complete their feeding alone, before pupating a metre or more above the ground, suspended by threads in a tall plant, shrub, hedgerow, or on a wall. At this stage they are vulnerable to predation by birds.
The number of generations produced appears to be controlled by the timing of emergence. Butterflies on the wing by the first week of August are able to produce another generation, but those emerging later are not usually reproductive. They may go into early hibernation or drift southward searching for suitable hiding places en route, whilst also stopping off at nectar sites on the way, to build up their fat stores in readiness for hibernation. Hibernation takes place in caves, hollow trees, dry-stone walls, and buildings, including barns and sheds. Problems arise when houses are used and butterflies fly in through open windows in late summer but can't get out in spring. They may also wake in winter when it is warm enough inside but not outside. These butterflies should not be released outside as they will die, but can be caught and put in a cool protected place like a shed or garage where they can resume hibernation.