Moving meat chickens outside
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Can you crack a window in your garage to get their brooder slowly worked down to the same temp as outside? Our garage doesn't get below 60, so we had to try to control the drop in temperature about 3-5 degrees each day. Definitely, at first keep the windows closed at night so it doesn't get too cold. Or you can move them outside if you can run the heat lamp out there for night time. Just remember Cornish X are very prone to piling up when they are trying to get warm, and will often suffocate the chicken on the bottom. But if you have a ranger or other meat bird you will probably be fine. And keep an eye on the "feels like" temperature! We had a difference of 8 degrees one night and had to bring all of ours back in to the brooder because we thought the low would be 40, and the feels like was 32!!
They can handle low temperatures, but you have to condition them for it first. Start with a tiny crack and see how much that changes the day time temp in your garage/brooder.