While animal bones and skulls certainly are not a daily donation, they aren't nearly as rare as they probably should be. I'd say we get a skull or two in about three times a year. This is a pretty good haul for one box. Nice variety here.
We also get other bones, teeth, claws, and shedded snake skins now and then. We've gotten two horse skulls on separate occasions (one just appeared when no one was looking), and the greatest donation of all time was a skull... a human skull. I shit you not. That is whole different and crazy story. Here is an older pic from the Junk Pirate archives of the old bone drawer: