Judging by the tracks and the proximity to the trees, I assume that a squirrel is making them. Sometimes, but not often, I find shells near the holes.
Because the squirrel is not around, I can't ask him:
How do you know where to dig? Do you follow your nose, your memory, or some pull of the earth's magnetic field.
And, what is your success rate? If I find five holes around the tree, am I to assume you found five nuts? Or did you find just one? Or none -- and you went to bed hungry?
And, did you notice, as I did, that the grass, under all that snow, sometimes shows hints of green?
NOTE: For those who like scientific answers ... Scientists aren't sure how squirrels find their stash -- whether it's scent, landmarks or memory. But grey squirrels scatter their nuts throughout an area while red squirrels create little piles of nuts. Thus, the grey squirrel helps re-seed the forest more than the red squirrel. And squirrel connoisseurs often say that the grey squirrel is better eating -- not as gamy as the red.