What crows do

There's a flat roof immediately below my office window covered with roundish pale-colored rocks that average maybe 1½ or 2 inches in diameter. Here in Chicago we have a plethora of crows. I just watched one alight on the roof and meticulously stuff something it brought in its beak between the rocks, and oh-so-purposefully heap, one by one, three or four more heavy little rocks on top, making a mini-cairn. In the jumble, from the angle of my desk chair, I can’t now distinguish the cairn from the rest of the rocks. It makes me wonder if the crow made the cairn to identify the cache later, or if the little pile was only incidental to its effort to preserve its treasure by the burial method. If the latter is the case, what marker will it use to relocate what it buried?