The pied wagtail is a distinctive dark-backed race of the white wagtail, Motacilla alba. Found mainly in the British Isles it is Motacilla alba yarellii. The origin of the wagtail's name is obvious but the reason they wag their tails is not known. They hunt insects just above the ground, usually on foot, and seem to prefer paved areas. Here they are usually on the drive or the roof of the house. Pied wagtails gather in large numbers to roost, naturally in reedbeds, but they also favour man-made structures such as hospitals, stations and supermarkets.
Listen to the BBC Tweet of the Day on pied wagtail here. Watch a video of pied wagtails roosting at Heathrow terminal 5 here. Read more than you will ever need to know about pied and white wagtails here.