Are you using a heat lamp on the chicks and poults? There are been massive deaths from using Teflon-coated bulbs that are not meant for poultry.
What are you feeding? Check the feed to make sure there is no odd odor or color from moldy feed. If the least bit of water gets into feed, it can mold overnight and cause poisoning.
If none of the above are a problem, then I would look for the possibilities the others have suggested, of a weasel or the gnats as a possibility. A necropsy might be helpful.