some hatcheries have different systems then others. it could be marked for either being a male or a female. if you can find out which hatchery your chicks came from you might be able to find out if they mark only roos or visa/versa. if you have a good feed store manager they could possibly let you know exactly what the mark means. I'd give them a call then you will know for certain what sex the little one is. some hatcheries use different colors or marks to tell the roos apart but some mark the pullets as well. it sure is cute!! I'd call just to make sure it's a roo marking.