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The ‘White Wagtail’ is a slender bird, 16.5–19 cm in length. Its average weight is 25 g and the maximum lifespan in the wild is c. 12 years. The nominate subspecies Motacilla alba alba is basically grey above and white below, with a white face, black cap and black throat. ‘White Wagtails’ are monogamous and defend breeding territories.
The most conspicuous habit of this species is a near-constant tail wagging, a trait that has given the species, and indeed the genus, its common name. In spite of the ubiquity of this behaviour, the reasons for it are poorly understood. It has been suggested that it may flush prey, or signal submissiveness to other wagtails. A recent study has suggested instead that it is a signal of vigilance to potential predators.