Your post immediately made me think of this:

Here is a link that might be useful: label mystery

My guess would have been similar to the link, some kind of bird. I could envision a crow, or one of our two types jay doing something like that here.

In our former house, I could hear a crow on the roof and stood in my bedroom watching one at the skylight there one afternoon, 4' length, east facing and sloping glass. The crow had picked up a shiny brass screw someplace, and was dropping it at the top of the glass, watching it roll down, then would hop to the bottom and pick up the screw, hop back to the top and do it again. Over and over. If only they would explain ;)

Could be squirrels.

While squirrels are my nemesis - I have yet to get a single, ripe apple off the tree I planted 34 years ago (and neither have the squirrels for that matter, as they pull/bite/drop the fruit when they are only golf-ball size) - the backyard is pretty much off-limits to them. It is patrolled at irregular intervals by the grand-dogs who hate squirrels as much as I do. Plus, I would see other squirrel 'diggings' or prints in the soft soil of the raised beds.

I thought it might be some kind of bird, and we have grackles in abundance. But why? Maybe the little pictures that look like the end product got 'em hungry....

In other "bird brain" behavior, a saw something amusing a few weeks back. Here, we are at the juncture of the northern and southern populations of robins. The northern population comes south for winter and the southern comes north for summer; the two populations overlap in spring and fall and they don't like each other.

We've got a birdbath next to the greenhouse that's the most popular avian spa around. Often, there will be a queue of three or four robins patiently waiting for the occupant to finish its oblutions. Usually, it's all surprisingly orderly.... But once the occupant left, another quickly dropped in before the next in line could hop up. The second bird took mighty umbrage at the impertenance of this line-jumper - and the fight was on. Of course, it's difficult *not* to watch a good fight, so the next three birds hop over to watch the fracas. Which leaves the way open for #5....