We add new chicks in all the time. We keep a heat lamp on one end, and they wander in and out of the heat as they need it.
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I leave it at whatever it is for the older chicks. The more chicks I have in the brooder, the less heat they seem to need. I don't go by 5* a week though. I just watch them and when they seem ready to have less heat, I change the bulb (I don't have any easy way to adjust the height). With a larger number of chicks, 40 in a 3x4 brooder, they were off of all heat by a week and outside by 3 weeks. Our winter temp in the house is 67*, and we've still had three $500 electric bills in a row this year, and they have done great. We tried the 5*/a week in our garage last year and they didn't need as much heat then either but did need it a little longer than in the house.One more question! Do I raise the temp back up to 95 with the new chicks? My current chicks are at 90 degrees going down to 85 on Wednesday. Not sure how to handle the temperature between the two in the same brooder.