Chickens dying

[A Bouldin]

I have 3-4 week old healthy Cornish Cross chickens dying. I find them in the brooder dead just hours after they are active and eating. No bloody poop. No fighting. Can't figure it out.

 

[boskelli1571]

I have 3-4 week old healthy Cornish Cross chickens dying. I find them in the brooder dead just hours after they are active and eating. No bloody poop. No fighting. Can't figure it out.

How much did they weigh? A healthy Cornish X is a bit of a misnomer since they are basically born to die within weeks.
It may be that they died from 'natural' causes such as heart or lung failure - how many have died?