We have had this a few times with Orpingtons Kbs_05. The last time it was a very tiny piece of feather, but we have had pieces of grass seed and cockerels after either walking into something or fighting. The worst hen went to the vets and was sedated so that she could have a really good look. Couldn't see anything at all, but flushed the whole lot and the hen was fine. Danger is if infection gets a hold as loss of an eye will result. It is difficult, or even impossible, to look properly with the bird fully conscious so I would recommend the vet with all haste. Salt water is the way to go normally, but clearly it isn't working this time.