Abstract: Egg oiling is a form of management ... Parental care by malefemale and female-female pairs of ring-billed gulls. Colonial Waterbirds 12:148–151. Coulson, J. C., and J. Butterfield. 1986.
If you've spotted a blue ring around the yolk, the explanation is straightforward. It's the result of overboiling the eggs.
Listen to Sean rhapsodising about the ring ouzel above... About The Ring Ouzel: Irish nature is facing a very serious extinction risk, like never before. Accelerated vastly by human hands.