I have raised several batches of the same birds and this time is the first time in this cold of weather. Normally at 6 weeks I will get processed birds weighing 3 1/2 to 4 pounds, this time they were 2-2 1/2 pounds. So the only thing different was the cold weather, did they expell so much energy trying to stay warm that they didn't gain the weight? I had two heat lamps on them at all times. What's your thoughts? Thank you!!