I am have a number of eggs showing up with a tiny dark speck attached to the exterior of the egg yolk. These are barred rock pullets that have just started laying. I am feeding layer crumbles, whole corn, scratch and some oyster shells in a separate feeder. They get plenty of fresh water. I have 24 pullets and three roosters. The speck is showing up in about 1 out of 5 eggs. Any thoughts?