I would flush the eye out well with sterile saline or eye wash, look for injury, debris, any pus, etc. Once it's flushed well then apply terramycin eye ointment (most tractor supply stores carry it) or plain neosporin 2 to 3 times daily. It could have gotten pecked, gotten something in there, or possibly a symptom of a respiratory disease. Hopefully it's one of the first two and it will heal up.