I know everyone has a different opinion on this, so I think what works best is what your chicken's are telling you. I have 20 chickens. At 3 weeks old I put them outside in upstate new york in their coop with two heat lamps that overlapped so they could huddle in the light of both, in one, or away from the heat. I placed the food and water outside of the heat so they had to run into the cold in order to feed themselves. This started the 21st of April when temperatures were consistently in the 40s, snow happened at least once, and below freezing temperatures were uncommon but appropriate for the season. At the 4th week one lamp was removed. At the 5th week all heating was removed. Temperatures still dropped into the 40's, but feathers were pretty well grown (except for the near-naked cockerel who still wore his downy suit and was sporting all of about two feathers on each wing) and the group had learned to make a huge chicken-pile in a corner in order to stay warm. This flock is all the same age, but ranges from a couple of Jersey Giants to a bantam cochin and white tailed black Japanese. They might have been uncomfortable from time to time, but they all survived and at 8 weeks I open the coop door in the morning, close it at night, and they are pretty much on their own for food and heat (but it's pretty much all 60's, 70's, and thunderstorms now).
I doubt this will work if you only have three or four chickens, but seeing as how I had large enough numbers for them to keep each other warm, I figured it was better to get them started training for their first real winter now so it doesn't take them, or me, by surprise.
Said pictures of the unfortunate rooster. I have him pictured alone and with his sister to show how feathered he SHOULD be (they are both Jersey Giants from McMurray). If this dude can survive 50 degree weather without a heat lamp, your babies are probably safe.
Oh also, don't tell him I posted these. He tries so hard to be studly and boss around all the ladies, but those two little feathers on each hand don't give him much lift so the ladies just fly up onto the roosts when he gets all uppity, leaving him alone on the ground with the frizzle who isn't much to look at, the polish who can't really see what is going on (current hobby- running into things), and the silkie.
