Hens go broody when you don’t want them to… and won’t go broody when you do. 
Most folks I know call them "wind eggs" - funny, I've had chickens for years and just got my first one last week. Usually, they are just whites without a yolk, but mine did have a tiny little yolk inside about the size of a pencil eraser. I took a pic, but dont' have my camera here today!
