Could this be the egg of a mourning dove (Zenaida macruora)? I have flushed a bird from an egg in this general location in previous years, possibly a mourning dove. (I was too startled to look closely. It did call in dove-like tones...) AllAboutBirds.org, from the Cornell Lab of Orinthology, says "Also quite commonly nests on the ground, particularly in the West" (7/2018).