I put the photo of the big brooder shed on here just to show you that I do know what I am talking about Tony. I'm sorry if I read your response wrong
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It's just that yes, there are a lot of people out there who think they know everything, and we have all seen the posts by new chicken keepers who have problems, not only with their own hatched/raised chickens, but also with what they have brought from breeders who do not take care of their flocks when they are being raised.
I have also raised just 2 or 3 chicks at times, not only in those plastic boxes that I showed earlier, but in an old kitchen wall cupboard and once when desperate, in a banana box, which I stuck together lengthwise to give the chicks a bit more running around room.
But when you said 'at least' 40 degrees, that did worry me. Even turkey poults only need 37.5C in their first week as they are more susceptible to chilling than chicks are in their first days.
As Joy and Kegs said, those first days are critical if you want lovely healthy chicks. I have raised some of Joys chickens in the early days before she found out nearly all her girls will go broody if they even see an egg, and Kegs is right. Katie Thear (RIP) was very well thought of in the poultry world.
What I will say to anyone just starting out though, is that you don't absolutely need humidity meters and thermometers to raise your chicks. It's good if you can, but as you will have seen yourself Tony, if they are too warm, they will move away from the heat, too cold and they will crowd under the heat lamp.
So the ideal temperature is that they are under the heat source, but not all crowded together. There should be a small gap in the centre of the crowd. This gives the chicks on the outside of the circle enough room to jump over the others to have an extra warm in the centre, then the others will move out a little to give that gap back.
If they are spread out around the outside of the brooder, they are too warm, so raise the heat lamp a little
If possible, its a good idea too if you have more than 3 or 4 chicks, to use something like cardboard to round off the corners of the brooder. Then if something frightens the chicks and they run to the corners, they can't suffocate each other. They will climb on top of each other to try to get into a corner and the poor little things underneath get suffocated to death.
Joy did get an electric mother hen just in case, but I don't think she ever used it. They can be good too, as many of them can be higher at the front than the back, and the chicks can go to whichever bit is warmest for them. But I have never used them, so can't say anymore about that.