4 1/2 wk chick eye problem
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Have you gently looked into the closed eye to see if she may have gotten a piece of bedding/dust in it? If so you can flush it with saline to see if that helps.
Otherwise, could be either neurological or genetic developmental problem or a vitamin deficiency.
Add some vitamins to her water if you haven't done so.
Is she eating and drinking well? If not you can mix some of her chick starter with vitamin water and make a wet mash. Chicks (and most chickens) love wet food.
It's hard to tell in the photo, but is there a small lump/bump under her eye?
Also what type of leg problems did she have prior to developing the new symptoms.
Here's a good list of different vitamins, what they are for and which products/foods contain them:
Edited by Wyorp Rock - 3/9/16 at 9:23pm
I'm sorry for your loss.
Sometimes it can be just a case of "failure to thrive" or some type of genetic/developmental defect.
If you are having trouble with chicks from the flea market or auction you may want to ask the seller more questions such as vaccination details, bio-security, breeding program, etc.
You may also want to consider what new chicks are being exposed to if you aren't cleaning/sanitizing your brooder between new batches of chicks. Dust, dander etc., can hold and "pass along" some illnesses that young ones may not have a strong enough system to overcome.
Also you may want to keep a record of when, where and who you got chicks from, what symptoms/signs the chicks presented, that way you can start narrowing down (hopefully) the cause of loss and make changes to prevent it.