Grey Wagtail
Motacilla cinerea

Usually found on rivers, and sometimes on lakes, these birds can be a little difficult to get a good photograph of. The constant wagging of the tail can make things very awkward if you have low light conditions, like the first shot, which is a young female waiting to catch flies for her chicks. The second is of a winter male.
 

  

1: River Kelvin, Glasgow, 11/06/2005.
2: Sandhurst Balancing Pond, 13/03/2006.