Okay, after waking up to another two deaths, including one of the fresh callows, I took action on your suggestions, and I think you guys are right on about a water problem.
I moved the nestmate into the outworld and will watch to see if they drink from it, but moreover I think I've been significantly underwatering the poor girls.
In the past I was topping up the nestmate with 3 or 4 CC of water a couple of times a week, which seemed fine, and over time I've probably gotten less diligent about that, in particular because I've seen it "overflow" from that amount a couple of times. BUT -> I think actually what may have been happening is that if I insert the tiny bent syringe tip at the wrong angle, the water is going out directly into the substrate rather than into the water tower itself and so they didn't actually have too much water, they had it in the wrong place!
Today I did the "bubble test," and heard nothing, then filled 1cc at a time until I actually overfilled slightly (woops) and could hear the test, which took ~10CC! Going to get more rigorous about checking by listening for that and dial in how much / how often I fill.
Edited by bootsfirst, April 27 2025 - 5:36 AM.