Thanks, Anne and MotorcycleChick.
I just got an email saying that the sad chick is now a dead chick.
I asked them to put Corid in all the water for the whole flock (2 tsp of the 9.6% liquid solution per gallon). I also asked them to put the chick in a plastic bag in the freezer so I can send her for necropsy when I get back next week.
I also asked them to lay off all treats except a little cracked corn for now. I had given a variety of treats (cracked corn, BOSS, dry mealworms, lettuce, dates, yogurt, garden weeds like grass, dandelion, bindweed) and asked them to give a bit to the birds twice a day since they are used to free ranging during the day but they are mostly cooped up for the 2 weeks I'm gone and I didn't want them to get too bored and start picking on each other. I usually give them some treats each day even though they do free range, just because they run up and beg when they see me coming, so the treats themselves aren't new (except the dates--I only gave out a couple of those before I left, and some of the hens seem to like them). But maybe they were too enthusiastic with the treats and the chickens could be getting sour crop? I did ask them to give plenty of grit each day, though, since the hens weren't free ranging.
I just don't know. I feel so awful, and worried about the others, and I wish I were there to manage it.