Hazel nuts are popular with many small mammals (our native dormouse, muscardinus avellenarius, is called the hazel dormouse for a reason). And the way a nut is opened can tell you who’s eaten it. This is particularly handy for telling if dormice are present, as you’re very unlikely to see one.
So, starting with the easy one. It takes strong jaws and teeth to split a hazel nut. So if you find a discarded nut shell split in two, or shattered, the nut has probably been eaten by a squirrel.
Bank voles also have big, strong teeth, so can bite rather than nibble through to the kernel. Quite often they will leave an irregular, roundish hole in the shell. They tend not to leave lots of teeth marks on the shell near the hole, unlike mice.
Now we get to the tricky part: distinguishing between nuts eaten by apodemus mice (woodmice and yellow-necked mice) and dormice. Both sorts of mice nibble neat round holes in the shell, so you need to look closely. A hand lens or magnifying glass helps.
A hole nibbled by an apodemus mouse will have teeth marks going down the edge of the hole (vertically). They leave more scratches on the outside of the shell than bank voles. Hopefully you can see both those features in this picture.
The inner edge of a hole made by a dormouse will be much smoother, as they gnaw around the hole, rather than down. But they do leave lots of scratches around the outside of the hole.
Another clue to help you tell the difference between apodemus and dormouse nibbled nuts is where you find them. If you find a cache of nibbled nuts, it’s likely to be apodemus. Dormice generally tend to eat and then drop nuts where they find them, so their nuts are often more spread out.
If you find a nut with a really tiny hole (maybe 1-2mm across), this has probably been eaten by an insect.
Here are a couple more close-up photos of nibbled nuts. Can you tell what’s eaten them? I will put the answer in the comments to this post. Let me know how you get on.
I’m starting to put together a page of photos of mammal signs. This will develop as I delve into my archive, and take more photos. I hope this will eventually become a useful reference (particularly as many books have line drawings rather than photos of these sorts of things, which can be rather hard to interpret). I’ll let you know when it goes up.