Motacilla alba (White wagtail)

Story behind:

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 study in 2004 has suggested instead that it is a signal of vigilance to potential predators.

The species has adapted well to human changes to the environment and has exploited human changes such as man-made structures that are used for nesting sites and increased open areas that are used for foraging. In a number of cities, notably Dublin, large flocks gather in winter to roost.

Source: Wikipedia