Is a brooder necessary for baby chicks? If you have a broody hen to take care of them, no. But if you are taking care of them, yes.
Ddawn, trust you to come up with a great link. I don't remember seeing that one.
I put my brooder in the coop from Day 1. It is basically a wire cage with a draft guard around it, much like a rabbit cage but 3' x 5' in size. I keep one area in the correct temperature range but let the rest cool off, usually about 20 to 30 degrees cooler in the far corners. The first couple of days they spend most of their time under the heat lamp, but by the third day they are playing all over the brooder, only coming back to the heat when they want to warm up. They do tend to sleep in the heated area.
If I tried to keep the entire brooder the exact perfect temperature, I'd always be very concerned. But by letting them find their own comfort level, my life is a lot easier. By the time they are about 4 to 5 weeks old, they have fully feathered out and I take the heat source and the brooder away.