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Clumping is when a large group of small birds gather together to roost, with the aim of retaining warmth. It is characteristic of very small species in winter or when young, examples being the wren (Troglodytes troglodytes) and the long-tailed tit (Aegithalos caudatus). These tiny birds lose heat more quickly than larger ones. In order to stay warm through the night, tiny birds sleep close together to catch body heat off the neighboring bird.