2 chicks died in 24 hours
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Welcome to BYC!
Sorry to hear about your loss.
Do you have any photos of your brooder set up and the chicks?
Did they come from a hatchery or an individual?
Did you examine the inside of the mouth/throat to see if there were any obstructions?
What type of food/treats are you using?
What does the poop look like?
Any actions before, trouble breathing, coughing, lethargic, puffed up, not eating/drinking?
It wouldn't be wry neck as that doesn't come on suddenly with quick death.
I agree, probably failure to thrive, but make sure there couldn't possibly be any toxins (moldy feed, etc.) The only thing that would drop them quickly with convulsion I can think of is toxin or poison.
Chances are simply failure to thrive if all others doing fine, the brooder is clean, food pure, heat right temperature.
Sorry for your losses. It does occur though with larger batches simply from stress of the group and transport.