A common and widespread butterfly across Europe, Common Blues can be found in large colonies. As with others in their family (Lycaenidae), the Common Blue can be found roosting communally, generally facing downwards, on the stems of grasses and other vegetation. The main theory for communal roosting in butterflies is that it may act as a predatory deterrent, and it is a strategy they can use overnight, but also at times when they are unable to fly due to low temperatures or poor weather.
Butterflies are reliant on heat from the environment, and use the base of their wings for temperature control. They will use different postures, from flattened to shut wings, and different behaviours, from basking to hiding in shady spots, depending upon their body temperature. I spotted this small group of Common Blues sitting out the cold and drizzle on a particularly unappealing survey day in southern England.