Answer:

Roup in some of its stages is generally the cause.

A slight cold may at times settle in the eye and cause it to swell. This is especially true if a fowl roosts near a crack in the wall of the roosting quarters where the wind blowing through will strike the head and eye.

Bathe the eye with equal parts of warm water and vinegar and annoint it with carbolated vaseline.

See question relative to roup.