Today only - she's hunched - separate from the other 5 - her vent does have sick looking poop on it (mucus-y and yellow) - otherwise her eyes and sear look clear and her legs look fine....
She's a 5 or 6 month old red sex link. She's no thinner or heavier than the other birds.
She just started 'acting funny' today - going off by herself, sitting down in the middle of the yard or under a tree (not her usual behavior). Hunches her back.
Everything that I know to check is copacetic - her eyes are clear - her comb is nice and red - her legs look healthy...except her vent had some mucousy, yellow poop on it.
How long has she been laying? If she has been laying, how were the shells? Do you think that the yellow is egg yolk? Have you seen her poop? A picture of her poop would be helpful. Can you feel any masses in her abdomen?
Whatever it is, it sounds like she's very sick. What are you willing/able to do? Vet, supportive care or are you going to cull her?
If she were mine, I'd would put her in a warm, draft free, safe place on clean towels with access to food and water. I'd also do a thorough exam (look in mouth/ throat, check for lumps, masses, cuts, bruising, mites/lice, etc), take her temperature and inspect her poop. I'd also dust her for mites even if I didn't see any.
I know birds don't show too many outward signs of illness until they get pretty far along - but I didn't expect 'very sick' (that sounds crass - I'm not trying to be : )
Since they free range around the yard I couldn't tell which poop was hers but I'll be sure to pay more attention tomorrow. Its possible the yellow was egg yolk.
They just started laying in August - and all the shells are fine - smooth, hard - I can't tell which one is hers, specifically, but there are no marked differences between the shells, anyway (my inexperienced opinion, of course).
I'll set her up somewhere comfortable tomorrow morning and try to examine her better.