I remember from when I was a child when I would play in the sandboxes we would make these "sand bowls" by pouring water on a mound of sand and working away the dry sand around the wet area. I am hoping to achieve a similar effect but with the help of the slough birds and (if any) their aesthetics. An immediate challenge would be figuring out what solution (water doesn't seem to promise longevity) that would be good to preserving these ephemeral marks on an object that I hope to ground in a more permanent space (by taking it out of its natural area and presenting/constructing it as an art piece).
A similar approach would be to use a pan/sheet of some sort (like a deep cookie sheet) filled with sand and see what the birds will make of it afterwards. Another challenge would be whether or not they would notice/interact with a foreign object near their grounds. Beyond their footprint, I am thinking of other ways to visually capture their mark in their natural space.