Went out to do chores this a.m., and there was a tiny egg by the feed bucket. My girls are only 3 1/2 mos., I thought they wouldn't start laying until 5 or 6 mos. I'm not sure which one is laying, but I suspect a Buff Orp. Is this common? or do I have an "early bird"? Do I need to provide oyster shell now? They get to free range for a few hours a day, get crumbles and scratch grains.