Re the nesting habits of Juncos, I have had my first experience this spring (just south of Everett WA). I was given a birdhouse for Easter and promptly hung it in my backyard from a garden pole, sheltered under a small tree. Within days it was obvious that a couple of juncos had taken up residence. There was a lot of activity, with the two of them taking turns bringing nesting materials to the birdhouse constantly. Then, about two weeks ago, in addition to the adults repeated calls, we could hear faint chirping from the newly hatched babies. They would go silent whenever I got too close - I felt like I was intruding, but I had to get some plant tending done nearby on occasion! The parents continued constant short trips, I assume to find bits of food for the babies.
About 5 days ago I noticed there was no activity around the birdhouse. I have been afraid to look in for fear of scaring them, but I think they have all "flown the coop." I read online that the babies leave when they're ready and that the adult male might stick around the area to claim the same nest for the next breeding season. (They have one or two broods a season.) I also read that the females, since they are not guarding the nesting area, will fly further away. For now, I have not seen either of the adults in days.
What I do not know is how this works. Will they be back yet this year? If so, do they inhabit the nest, male or female, or both, at any time once the babies have gone off on their own.
I find this fascinating and am having my daughter, who built the birdhouse for me, make a few for us to give as Christmas gifts for close friends so they can enjoy the experience also.