What does your coop look like as far as draft protection? They don’t need a draft hitting them while they are asleep. My brooder raised chicks normally start roosting at about 10 to 12 weeks, but I’ve has some start at 5 weeks and some take a lot longer, so we are probably talking draft on your floor for a few more weeks.
Are they regular chicken breeds that look and act like chickens or are they some special exotics, those walking toilet brushes or ones that have funny things on top of their heads? I don’t deal with exotics so I don’t know how fast they feather out. Also, are they bantam or full-sized? Bantams might be a bit more delicate since they are smaller. Same thing is true for full-sized chicks that hatch from those tiny pullet eggs. Those chicks are going to start out a lot smaller than chicks that hatch from regular eggs, so if you have some that are really noticeably smaller than most of the others (considering breed) those are the ones most at risk.
Do you have adult chickens out there so you have integration issues or are they going to be the only inhabitants?
Do you have electricity out there and a safe way to provide heat? You sure don’t need a fire out there. If you can provide heat and don’t have integration issues, they could have gone out three weeks ago. Mine are in my brooder in the coop as soon as they come out of the incubator.
How well are they acclimated to the cooler temperatures they are going to see? Have they been exposed to cooler temperatures or have they always lived in the tropics? Chicks feather out faster if they are exposed to cooler temperatures plus their bodies can just plain handle it better. Going straight from the tropics to cold temperatures can be a shock.
I can’t give you a magic number but I can tell you some of my experiences. I keep a fairly large brooder in the coop so they are protected from the adults. The chicks go in there straight from the incubator. I only heat one end of that brooder and let the rest cool off as it will. In really cold weather, like days below 50, you need to heat a little more area with very young chicks (or cover it so more stays warmer) so they can get to the food and water without it being real cold. But I let the far parts cool off as it will. Unless it is the middle of the summer or winter, those chicks normally spend the first couple of days near the heat but are soon roaming all over that brooder.
In the middle of summer in a heat spell, I once turned the daytime heat off at 2 days and the overnight heat off at 5 days. By their actions they were telling me they were too hot, even in the far parts of that brooder. In the colder parts of the year, I’ve kept the heat on for up to 5 weeks. But that is only heating one small area and letting the rest cool off so they do get acclimated.
I’ve moved chicks to my unheated, draft-proof grow-out coop at 5 weeks when the overnight lows were in the mid 40’s Fahrenheit. That’s about +5 C. I’ve had 5-1/2 week old chicks do fine when the overnight low hit the mid 20’s F. Like you, I had enough that they could keep each other warm.
In the summer, I’ve had broody hens wean their chicks at 3 weeks so they are on their own at night with lows hitting the lower 70’s. I’ve had broodies wean their chicks at 4 weeks with the lows in the mid 50’s. I had a broody take her chicks to the roost at night where some were not warmed and the overnight lows were in the 70’s. All these chicks were well-acclimated.
I don’t know what specific conditions you are facing in the next few weeks or much about your set-up and chicks so I won’t give a magic number for you. There are just too many things to consider that I don’t know. But I will tell you those chicks can be a lot tougher than people give them credit for.
Good luck and welcome to the forum!
