My chicks are a week old, and the lady I got them from had them under a heat lamp. I checked on them last evening and they were standing in a corner in a cluster. They were not peeping, but I could tell they were starting to get chilly as they were all puffed up.
One by one, I put them under the cave on the heat mat and held them there for a few seconds so they could feel the warmth. They all ran back out to the corner peeping, trying to get under the other peeps to get warm.
Then I realized, they were looking for the heat LAMP, to them heat had to be light. I turned off the lights directly overhead the brooder so that it was a little darker in there. That really got them distress peeping. Then, one by one I put them under the cave on the heat mat. 2 minutes later, no distress peeping, no pig pile in the corner trying to get warm, and no light to keep them awake and active.
This morning, they were out and about. They had finished off the food that I left last night and were darting back under the cave to warm up, then popping out to scratch through the shavings looking for more to eat. Happy little peeps.