Last year we had a very grey Yellow Wagtail around the sheep pen on the way to Paddy's Pool Hide and we have another one this year. Have put some photos on to a yellow wagtail topic on Birdforum just to get a range of thoughts. You can follow the discussion on there, if there is any, or start you own with the attached pictures here. The last two pictures were taken last year and the first two are this years bird which is best seen around Paddy's Pool Hide
http://www.birdforum.net/showthread.php?t=138719,
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I saw two like this on the grass by the overspill carpark at Flamborough Lighthouse on Monday and they had me scratching my head.