Cornish cross chicks dying
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I'd suggest access to food 24 hours a day. Check your brooder for drafts as little chicks are susceptible to drafts. I'm also wondering where you live and if this is your first batch of Cornish Cross chicks. I ask because I've given up on them. I've had great results with Freedom Rangers and Red Rangers and layers, but dismal results with Cornish Cross. I live at 8,000 ft of altitude and I believe that this may be a contributor since CX have heart issues. I've also never tried brooding baby chicks at this time of the year, and wonder if your shippment may have been subject to cold temperatures in transit and may be compramised in some way.
At that age, there's no need to restrict food. Worry about that in weeks 3+.
In my experience, CC's have very weak ability to digest coarse feed. That, combined with restricting feed, is probably causing them to get their crops bound up. Give them 24 hour free choice, and avoid bad feed (Dumor!) and they'll stop dropping.