The pied wagtail (Motacilla alba yarrellii) is a real trial to photograph as it just does not keep still! It is always running around, here and there, chasing this, chasing that! It is, in fact, the smallest bird that actually walks; other small birds tend to hop. You quite often see them running on tarmac or concrete (ie hard surfaces) rather than grass, I guess it is an easier surface to walk on!
The other interesting thing about this bird is that it is almost indistinguishable from the white wagtail. In fact, the British pied wagtail is a sub-species of the European white wagtail being just a little darker in colour. It takes an expert to tell the difference but apart from the odd 'white' that turns up on migration, the ones we see in Dorset are almost certainly going to be 'pied'.
In terms of a garden bird, this was once an almost certainty in many gardens but, sadly, like so many other species this is no longer the case. We never get them in the garden itself but we do see them in the road outside and, despite the abundance of food we put out it does not seem to interest these little chaps! Wagtails are basically insect eaters anyway and in winter there are very few insects in gardens.
My father always called this bird 'Polly Dishwasher', the long tail making it look like the old long handled dish washing utensil that adorned kitchens and sculleries until the 1950's.