Like this Yellow-rumped Warbler, I found laying beautifully in the yard. I could tell it was not killed by any of my cats or dogs because it remained in full, nearly pristine condition.
(Click on any of the images for a larger view.)
The thing about bird remains is that they can stay outside for YEARS and you still may never have a clean skull.
Perfect example being this cardinal head I found long ago that I've had in my "bone box" for all this time.
It just looks more weathered, rained on, and flat now if anything.
So I've decided to hang the Yellow-rumped Warbler remains appropriately over my pet cemetery, from an Egyptian scarab beetle amulet instead. I'm hoping the feathers with slowly wash away leaving a beautiful skeleton.
If it works, I'll have follow-up photos.