Sorry, having trouble using quotes on this forum, but this in response to a couple of posts on page 1 explaining about weaning chicks off supplemental heat. Sounds like we need to start doing that! I had no idea that providing too much heat will slow feather development and potentially reduce long-term cold hardiness. I read somewhere about the danger of chicks getting pneumonia from being chilled if they are not fully feathered on the back because their lungs are supposedly close to their back, so that's why we've been keeping them warm. We have been reducing the temperature in the brooder but it sounds like we've been doing it slower than we should have been.
We'll move them to the sun room and gradually raise the lamp until they no longer need it. We'll also let them spend more time outside. Our coop won't be ready until at least a week from now, more likely a couple of weeks. That should give us enough time to acclimate the chicks to colder weather so that by the time they go in the coop they will only need the lamp at night.