Birds of a wide range of species show characteristic movements of their tail, often called tail flicking, tail wagging or tail flashing. Tail flicking refers to vertical up-and-down movements of the tail, while tail flashing is defined as a horizontal movement, often including tail spreading. Here, I review proposed functions of such behaviour. Most relate to communication with conspecifics, predators or prey. Tail flashing may induce movement of the bird's prey that makes the prey more vulnerable to capture (‘prey-flushing’). Tail movements may signal to a predator that the signaller has detected it (‘perception advertisement’), or that the signaller is particularly alert or otherwise difficult to catch (‘quality advertisement’). Further, it may warn conspecifics of predators (‘alarm signal’), or it may advertise quality as a mate, signal social status, or aid in flock cohesion. This behaviour may, possibly, though it seems unlikely, represent a cue rather than a signal in that it benefits the receiver, but not the signaller. For each postulated function, I develop predictions stemming from that function, and interpret the available empirical evidence in the context of these predictions. I finish by synthesising our current state of knowledge and by identifying the future empirical studies that would most improve our understanding of this widespread but unjustly neglected avian behaviour.