The vet was clueless. The chicken's been sneezing too. He saw no real redness or irritation in her throat (good news) and the fact that she's been sick for a week and the other chickens haven't caught it rules out an awful lot of things. He gave me antibiotics to give her 2x/day for a week (the chicken hates it!). He also noted that she's thin and recommended general TLC. I thought about keeping her indoors (it's chilly out) I think she'll feel the least amt of stress if she's outside. I'd love to keep her away from the other chickens, and especially keep her poop away from theirs now that she's on antibiotics and their poop will be fertilizer in the garden (I'd prefer to discard the antibiotic-laced chicken poo) but I don't have anywhere suitable to keep her inside and the cats and dog, mostly the dog, will probably stress her out even worse. I fed her a snack of a bunch of worms as a treat after the vet trip, and I plan to keep giving her worms every day as a treat while we do these antibiotics.