In a back corner or a front corner?
I found a similar pile around the entrance front corner. Turned out that on warmish days, the local bluejays and cardinals found that the mouse guard restricted traffic to the point that they could perch on the landing board and eat bees as they came out one or two at a time for cleansing flights, without getting swarmed by angry defenders. They are also smart enough to realize that in 3 feet of snow, the dogs will get stuck in drifts and be unable to chase them off effectively. Wouldn't have believed it if I didn't see it myself. I knew corvids were smart birds, but this takes the cake.