In reply to Mark Collins:
Definitely an immature Pied or White Wagtail (Motacilla alba / yarellii), NOT a yellow or Gray.
In winter Pied wagtails, especially immatures, have a yellow or even greenish tinge to the 'white' parts of the head.
The defining feature is the bib - immature Grey wagtails have no dark marks under the chin, while adults have black all the way from the beak down to the breast. Immature Yellows have a thin dark necklace, while adults (and also Citrine wagtails) do not have a dark bib. Only a pied wagtail has the white chin with a dark crescent (or inverted triangle in juvniles) beneath.
Also a Grey wagtail would have pale legs.
Post edited at 08:29