From all of my research/internet browsing, I am seeing that most people let their chicks out and 5-6 weeks without any heat lamp, but most of what I am seeing is people writing around June, July, or August, when it is warmest. I am new to chickens, so I am a bit jumpy to let them go without a heat lamp due to my inexperience.
I have 13 chicks that are 5 weeks old. Since they were 2 weeks old, they have been in a 3x5x2.5 ft cage that I built as a post brooder box. This cage is in a shed outside, with 2 good heat lamps hanging about 12-15 inches above my chickens heads now. I am in southern AZ, so the daytimes are extremely warm (generally low to mid 80s). The night is averaging about 48 degrees or so, but it still sometimes gets down to 40. Is 40 too low of a temperature for these chicks to be out in without a heat source?