From Heyjude in the comments:
OK I blew it up - and sharpened it and cropped it, etc and the only thing I see is that there's
a bit less foreground grass in front of the indicated part. Now if you are going to claim the rounded portion is a head - then what about the 2 shorter ones behind it? and maybe a less distinct short one in front of the tall one? Nah! take away the foreground grass and you'd have one large dark section like to the right of that. I can't figure out a way to add the 'enhanced' photo I emailed you. Perhaps you could post it?
Duly posted - Heyjude's enhancement from the original.
It's a possible explanation. I'm not sure I agree with this particular one but it's important to consider all possibilities. We have another bout of snow in the UK so I haven't been back to try again - the snow isn't the problem, it's the lack of sunlight. I need to get the conditions as close as possible to how they were when I took the original picture.
Anyone else want to rip into this one? If someone knows of a natural explanation, or a possible explanation, I'd be interested in hearing about it.