could be anything something they ate, something they stuck their head into and got nicked and it grew from there could be some kind of bug or fly then it again it could have been the easter bunny. All jokes aside that is only the 3rd time i have seen it and that one isnt the worse one the worse one i seen both eyes was completely swelled up and couldnt see out of and she was feeding it couple of times a day with a syringe to keep it alive. We cut both bubbles and got all of the yellow goop out and filled it with neosporin and put it on duramycin and about a week later swelling went down and chicken is still alive to this day without any other problems. Knock on wood