Two days ago another, smaller snake came and ate all the babies. One adult House Wren mobbed it over and over, and a couple of Anna's Hummingbirds also hovered around the scene. The fact that only one adult wren was mobbing the snake makes me think something happened to the other parent as well as the babies.
This morning I opened the nest box, looking for jumping spiders or their egg sacs, and I found the snake still there. At the moment it's torpid and sort of upside down, so I'm not sure of the i.d.
The reason I was looking for jumping spiders is that House Wrens (mostly males) collect jumping spider egg sacs and attach them to the walls of the nest cavity, and the sticks that make up the nest. When the spiderlings hatch they go around gobbling up mites. (At least that's been observed in nests of captive House Wrens). I was trying to figure out what kind of jumping spiders they collect.