You have probably come across ladybirds clustered under leaves or bark during winter. To spend the winter, seven spot ladybirds - otherwise solitary creatures - they seem to actively seek each other. I took the photo above a few minutes ago in my garden. I counted 16 ladybirds - most were 7-spots, with two Harlequins - on the shady side of an agave killed by this winters' harsh frosts. Before I go on to explain this communal overwintering behaviour I have to explain why ladybirds are so colourful. Ladybirds are aposematic, a term describing an antipredator adaptation by which organisms have evolved bright, contrasting colours (think on the yellow and black stripes in cinnabar moth caterpillar or wasps) to warn predators of dangerous behaviour (stinging) or distastefulness. Ladybirds belong to this later group. Their beautiful glossy red and yellow elithra with black spots is the first line in a complex defence system, a warning signal to predators, probably birds, of their foul taste. Their bodies contain a bitter tasting alkaloid. If the predator ignores the warning signal and attacks the ladybird - or if you handle them a bit roughly - they release a yellow liquid from their leg joints, with high concentrations of the alkaloid, coccinelline, in what is called "reflex bleeding".
Given that ladybirds can secrete up to almost a quarter of their body weight during the reflex bleeding, it is an energetically demanding defence mechanism. During the winter, when ladybirds do not feed and need to save precious resources, they do not reflex bleed, although they still taste bitter. It is because of this that they cluster together: as other distasteful prey that lack a mechanism to let know of their foul taste to a potential predator, clustering allows individuals with warning coloration in a group - even if they are unrelated - to benefit from just one of them being injured or killed by a predator, as the predator is unlikely to attack further ones in the cluster.
Holloway, G., Jong, P., Brakefield, P. & Vos, H. (1991). Chemical defence in ladybird beetles (Coccinellidae). I. Distribution of coccinelline and individual variation in defence in 7-spot ladybirds (Coccinella septempunctata) Chemoecology, 2 (1), 7-14 DOI: 10.1007/BF01240660