I don’t provide that much space per chick in a brooder. That sound strange coming from me because I always advocate for as much space as you can provide. I’ve kept 28 chicks in a 3’x5’ brooder until they were 5 weeks old. They were OK but were getting a little tight. I also kept 21 chicks in there until they were 5 weeks and they were getting tight. The first group were almost all female, the second group almost all male. Sex does make a difference.
My current brooder, 3’x6’, is permanently built into the coop. The top is my droppings board. I keep heat in one end and let the rest cool off. Last February when I put 21 chicks into it straight from the incubator, the overnight lows were below freezing. I had it wrapped so one end stayed pretty toasty but there was sometimes frost on the far end. The food and water was on the toasty end. I don’t use a thermometer to tell how warm it is in there, just warm enough is good enough and they will find their comfort zone. After a couple of weeks that was all over that brooder during the day when it was warmer. At night when it was cold they stayed in the warm end.
My chicks are raised so they acclimate to the cold. I’m convinced they feather out faster that way. I have had chicks spend the night with the lows in the mid 20’s before they were 6 weeks old with no additional heat. I don’t advocate that going straight from a tropical brooder to freezing temperatures at that age. Try to acclimate them if you can, maybe by exposing them to cooler temperatures during the day. We all do it differently so it’s hard to be real definite as to when they can go without heat. I have kept heat until 6 weeks in truly cold weather. I have turned daytime heat off at 2 days and overnight heat off at 5 days in a brutal heat wave a few years back. Each brood is different.
What you need in a brooder is food, water, protection from predators, protection from the environment, and almost always a heat source. The first three should be pretty straightforward but make sure the water is clean. Don’t just add water to the dish or whatever you use. Dump the old water out if it gets dirty but in all cases dump it at least every two days. That stops the lifecycle of certain dangerous microbes and also mosquitoes. Keeping the water clean is extremely important.
By protection from the environment I mean you need to keep drafts off of them. A breeze hitting them can chill them so wrap the sides or use something to keep breezes from the lower part of the sides. They need fresh air to breathe. Ammonia will develop from their poop and ammonia is lighter than air. Openings above them will allow that dangerous toxic ammonia to escape.
It needs to be dry. A wet brooder is a dangerous brooder. A wet brooder breeds smells, creates toxic gases, and breeds deadly microbes. Keep it dry.
That 90 to 95 the first week and dropping it 5 degrees a week is kind of a dangerous guideline if you try to keep the entire brooder that temperature. Heat is more of a threat than a little cool. What they need is a place to go to if they need to warm up but also a place they can go to so they can cool off if they need to. Try to keep one area those warmer temperatures but allow the brooder to cool off a lot more further away. Twenty to thirty degrees cooler is no problem as long as they can go warm up if they need to. That’s my biggest concern with those plastic totes, aquariums, things like that. It can be hard to get the far end cool enough. One thing you can do is to put something in there they can hide behind to get away from direct line-of-sight of the heat. That can help them cool off. That’s why I don’t advocate a certain area per chick in a brooder. Instead I want one big enough they can get away from the heat if they need to, no matter how many or how few chicks are in there.
I’ve probably made it sound complicated. It’s not. Just concentrate on the basics and you’ll do fine.