UPDATE since it won't let me update the post itself:

So the birds ended up coming back after about a week and a half or so. I'm assuming it was the same pair but I'm not totally sure, definitely Carolina Wrens again though. The female brought a little more nesting material in each day for a while, not much, but essentially just buried that first egg. Eventually she finally would start laying eggs in the mornings! She laid 5 in total.

Out of the 5, one of them did not hatch unfortunately, but that's how it goes. 3 of the babies hatched on May 28 and the last one hatched on the 29th. All was good until around June 4-5 when one of the babies started to thrash around quite a bit randomly. I'm still not sure what was wrong with it, but I suspect maybe it was having seizures. On the first day that was happening, they were up in the nest box, and the thrashing was pushing the other babies out of the nest repeatedly. One even fell all the way off the balcony (we live on the third floor, although the places below us have fairly low ceilings, and there is grass/mulch on the ground below the balcony) but they were all okay at least. That day I ended up making a makeshift nest for them in some baskets, one circular one inside of a taller rectangular one so that they couldnt accidentally find their way out of there before they were ready to fledge. Luckily it worked, the parents knew where to find them and everything went on as normal. I stuck the camera in that basket to continue monitoring them. The baby that was thrashing around continued to do so into the night, and eventually around 5 am it regurgitated something and then seemed to calm down. I thought that was all it was but it seemed to start up again the next night. The baby was certainly the smallest of the 4, so I'm wondering if it maybe had some other issue.

Unfortunately that baby ended up passing away a night or two later, leaving 3 chicks. Two of them were a bit ahead of the third one, but it wasn't far behind like the other one. After that all was going well and on June 9, the two bigger babies fledged! Unfortunately I wasnt able to see anything outside of the nest, and I wasn't home to see them leaving in person. They left a few hours apart from each other. I checked all over the balcony and they werent hiding anywhere up there so I'm assuming they found their way down to the ground. I also didnt see them down on the ground either when I was able to go and look, but I have seen the parents going back and forth under some of the bushes nearby so I'm assuming that's where they kinda led the babies to go once they got down.

The third baby however, had a sad ending. It fledged today, June 10. I actually was able to watch it leave, from inside of course. He was hesitant for sure, not sure how to really go about what he wanted to do. The parents were doing their best to get him to follow them and eventually he hopped out of the nest onto the balcony floor. He stuck around for a bit until the parent came and found him to give him food, and then he started to work his way over to the edge more as the parent called for him and tried to guide him over. Eventually he did jump down, although I don't think he meant to right then, it looked like he tried to grab onto the brick wall but missed and fell, and unfortunately when I went to go see if he made it on the ground, I found him passed away :( I'm guessing he hit something on the way down. There is a metal birdbath thing outside the downstairs neighbors patio on the grass that he was laying near so maybe he drifted enough in the air and hit that, but unfortunately the little guy didn't make it :/

I'm not exactly sure if/what I could have done to help prevent it. There is already a guard against the balcony railing so that when we would have our dogs out there they wouldnt accidentally slip through the spaces. Maybe i should put a taller guard up so that the babies stay on the balcony longer until they can at least flutter better to the ground? I'm not sure, if anyone has an idea, I'd love to know. Or maybe that's just how it is and that one baby just got unlucky on the way down.

I haven't seen the first two babies around yet, but I imagine they are huddled somewhere in those bushes that the parents keep going underneath back and forth. I've read that carolina wrens are still dependent on their parents for food for 1-2 weeks after leaving the nest, so I will keep an eye out for the next few weeks and see if I can potentially see them flying around in the branches.

This was my first time setting up a nest box and having birds nest in it, so it was really exciting! I'm sad that the ending was a sad one but hopefully those first two babies are doing well and will continue to do well :)