During our visit to this block of Collingwood, between 21st and 22nd Sts., we ask three residents which area neighborhood they most closely identify with. We get three different answers. 1: “Noe Valley – although I guess, technically, Noe Valley’s northern border is 22nd.” 2: (Shrug.) “I don’t know. Maybe the Castro? Eureka Valley? What is Eureka Valley? I suppose you can’t really call this a valley, since it’s up on a hill.” 3: “Do you live around here?” Not quite the consensus we’re hoping for. But surely we can all agree that the 400 block of Collingwood is San Francisco’s preeminent home of sidewalk birdhouses....