We have a mother robin that has made a nest and laid 4 eggs in our wreath by the front door. One of the eggs was broken on our porch the other day. I think this either due to our neighbor's cat, or the black snake I saw by the porch a couple days prior. Thoughts @birdman? Fortunately, the other three eggs remain, and are the cool classic shade of blue.
Hard to say. It's highly unlikely that a cat or a snake would only take a single egg, drop it and leave the others. Cats would destroy the whole nest and snakes would very neatly and quietly eat them all. My guess would be a cowbird was interrupted trying to parasitize the nest. They show up, toss a host egg then lay their own, so if mama returns before the cowbird can squeeze out the egg you'd end up with a broken egg on the ground next to the nest and everything else looking normal.