What's their heat source? Any changes there?
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If you lose more you should have a necropsy done. Let me know where you live and I'll help you find a veterinary diagnostic lab. If you are in the US, many states offer free or very low cost necropsies for backyard poultry.I looked them up, and the heat lamps are not coated.
The brooder is in an uninsulated detached garage. It's open topped, but I cover it as needed depending on ambient temperatures and duckling behavior. And again, they don't show signs of overheating or chilling. Some of them are dying right under the heat lamps, while some are dying off by the edge of the brooder.