Cold brooding refers to brooding WITHOUT a heat lamp.
It may include brooding in the summer only when temps in the house or in a contained area outside are high, or use of a pre-existing heat source in the house such as a woodstove, radiator, or heater vent for the first few days, but where the temperature is typically NOT 95 degrees to start, but much lower.
After many experiments with cold brooding, we have a system that works, provided there are sufficient chicks in the brooder, or there is a nearby moderate heat source (heater vent or something like that) for the first 48 hours.
MINIMUM CHICKS: You need at least 4 chicks to huddle together to be able to warm themselves sufficient for cold brooding to work.
MAXIMUM CHICKS: More than 12-15 chicks increases your chances of having deaths due to piling.
UNIFORM SIZE: Brooding different sized birds increases deaths from piling. The bigger birds will simply sit on and smother the smaller ones. Avoid combining bantams with full size, quail with chickens, turkeys with chickens, etc. You can combine birds that are up to 1 week apart in age, but more than that will also increase problems.
EVEN FLOOR: If you have hollows or dips in the floor that are deep enough for chicks to get down in to sleep, they'll do so. They like a hollow to plump down into - and if they do, others will simply pile in on top of them, potentially smothering the ones in the hollow.
HARDY BREEDS OR SPECIES: Hot weather species or breeds don't do well with cold brooding, unless it is in a very warm climate. Guineas require a higher heat than chicks, and they don't huddle so well - they are a tropical breed, and they don't have very good instincts to stay warm when they are young.
This is how we've done it.
We use a plastic bin, with a layer of newspaper covered in about 1" of sand, gravel, or dirt (depending on what we have). If we use sand or gravel, we toss some dirt in also, to expose them to the microbes from the dirt - helps give them natural probiotics and some pathogens that they build resistance to (this reduces risks of various soil-borne illnesses).
The chicks scratch in the dirt or gravel, and pick up grit naturally this way, and we just scatter feed on top of the brooder floor (this is how we feed our chickens outside also, scattering their feed, we use no feed dishes except for things that are very liquid). They scratch around, wiggling their little fluffy butts, looking like toddlers playing grown-up. Very cute.
Sometimes I use chopped dried herbs to give them greens in tiny bits, and since they run under whatever is being dropped, they end up getting seasoned instead of getting fed! I break mealworms for the first few days only, then feed them whole, and tossing in a couple of pinches of those just gets them chasing each other for the tasty bits.
We put a huddle box in at the start.
A huddle box is a box that has had one side cut out, and turned upside down. Usually a pretty small box, just tall enough for the baby chicks to run into - ours was 7X7X7 to start, and then we cut it to be only about 5" high. The box contains their body heat, and they use it instinctively.
We line the inside with reflective bubble insulation, to provide a little additional radiant warmth. Gotta make it small, this size will hold a dozen chicks, or even 15 or so, they group together under it. If you make it very big, it fails to hold the heat, and it is nearly impossible to fit it into a plastic bin or brooder box without it taking up too much of their space - they'll be tripping over the waterer trying to get out of it if it is too big.
We use a mini-brooder with a short huddle box for chicks just out of the incubator. Once they are three to five days old (the age you'd usually get them if you ordered them), we move them to a larger brooder with a huddle box that is a bit taller (about 1" higher).
We use the huddle box continuously in the mini-brooder, and for the first 3-5 days in the standard brooder. After that, we use it only at night for another week, and then the huddle box is taken out completely.
We also leave them near the heater vent only for the first week in the big brooder, and then move it about 6 ft away. We move the brooder each week, to successively cooler locations, ending at about 4-5 weeks on the enclosed back porch, then we move them out in the daytime, and in at night for a week, or two, depending on temps and feathering stage (we put them out in the coop when they are fully feathered - they go as a group, and have learned to huddle together for warmth).
When we have chicks hatching over a period of a week, we go by the age of the youngest chicks to make changes in brooder conditions.
We have learned that groups of more than 12-15 chicks do not do well cold-brooded - they may have issues with piling and the ones on the bottom will smother.
We divide up larger groups into multiple brooders.
We have also learned that when you put turkeys in with chicks, the chicks may crawl under the turkeys when the turkeys are standing and the turkeys will sort of settle down as they get sleepy, and can crush the chicks if you have very many turkeys in with the chicks. Sad, because chicks can teach turkeys to eat, and some turkeys won't learn to eat without something to show them. Putting bantams, or quail, in with turkeys would be a really bad idea, for this reason.
It is also not a good idea to leave large depressions in the floor of the brooder if you are using sand or gravel. When you put it in, smooth it out clear to the edges so there is not a drop off in a corner, because the chicks may huddle in the depression, and others are more likely to crawl on top of them - smothering again.
Our chicks that we have raised cold brooding are active, very busy little creatures. They huddle together to sleep, and make a lot of noise all day - just peeping, not crying. They've been healthy, and they've transitioned out to the outside coop very well.
Cold brooding has been a success here, though there have been some learning experiences, and some losses because of it. But I'd not go back to a lamp - no way. The chicks are just so much more vigorous and active, and they are hardier, and there is a lower risk of fire when you do not use a heater.