I have a dozen guinea keets that are going to be 6 weeks old in a few days. I've heard that after 6 weeks (fully feathered), they're extremely hardy. I live in Upstate NY, so the winters here are cold and we have a lot of snow right now. I'm weary of putting them outside anytime soon. They're still in the brooder with a heat lamp. At what age do you think I can put them in the coop outside with no heat lamp?