wahay, you beat me to it Bella. i was just about to suggest a Yellow Wagtail.
Pied or White Wagtail:
You might have seen these guys around (we get a lot in town) they are called Pied or White Wagtails, although i call them bobtails some times. The yellow ones would act near enough the same.
I did look at the Yellowhammer but i think its wings look darker and its chest isnt as bright as the bird in your photographs.
i have never seen a Yellow Wagtail before. they are pretty
Sorry about the quality of the pictures. had to photograph the book because my scanner doesnt want to work today
