I had thought the male might be feeding an incubating female as it repeatedly flew to the nest box, thrust its head inside, and also completely entered the box for a few seconds before exiting. But it was not carrying any food that I could see, there was no sign of a female, and as I later read in Bill Bousman's Breeding Bird Atlas for Santa Clara County, this was nearly a month before the earliest reported nesting date of the species in the county. It turns out this is a standard courtship display for Western Blueirds; Birds of North America Online reports that "during courtship, males ... perch on box front [and] repeatedly poke head into entrance."