As long as it's not below freezing you're fine. Keep in mind that most of this stuff is below grade down in the ground. The temperature of the dirt is probably 50*. Even if it gets down to 32* at night for a short time, it's not going to freeze the concrete. Concrete takes about 28 days before it is close to fully cured. I would wait at least a week before attaching the bird house. If you're worried about the temperatures, throw a piece of plastic over it at night. You don't need to use calcium.
