Jutta Burger spotted this half shell of an egg under a sage bush on the southern edge of the forested grove on the west side of the park. She believes it’s the remainder of an egg that hatched naturally, not one that was raided by some predator. I looked it up. It’s almost certainly the egg of House Finches. White or the slightest bit pale blue, with occasional small black spots.
My ID was helped along by the loud and cheerful activity of a whole little flock of House Finches in the immediate area. One of them supplied the sound track to my video of the European Starlings, which are also breeding in that clump of trees.