My chicks are about 6 weeks old, and have most of their feathers. I let them out this morning (40F) and they were happy out all day--high of about 60. I have them in a chicken tractor that has good shelter from wind/rain. To my untrained eye, (first timer) they have all their feathers except on their heads. The low temps here are about 35-40 degrees. When can I leave them outside without heat? There are 9 of them, and they snuggle together to nap, but otherwise are active outside. They fluff out their feathers in the morning, but are active, scratching, eating, & happy. Thanks!