Transition stress can be hard on chicks...movement from one place to another.
Usually it is latent bacterial infection in the gut left over from hatching. (The process of hatching itself can create the environment for a slowly closing abdominal cavity to harbor bacteria).
Also, pure bred chickens, especially those with the more unusual features, have been selected for appearances more than hardiness. I find my purebreds to be the most temperamental.
The only other thing to watch out for is coccidiosis, which can crop up in young chicks with transition stress.
While a little young, if the chick did not have a strong constitution, and had been exposed to the oocyts on the breeder's property, it may have kicked in.
Watch your other chicks. If any of them begin to stand around looking lethargic and ruffled, it may be coccidiosis (a protazoan overload). Medicated feed is a good thing to have them on to prevent overgrowth until they are old enough for the bodies to naturally handle it.
The other thought is make sure your brooder temps are good...neither too warm or too cold. The heat lamp should be located in a place where the chicks can easily reach but leave if they get too warm. Do not have the lamp over the water bowl as that can create bacterial and coccidial growth.
Sadly, some just are not strong enough to survive.
I'm sorry for your loss of Copper.
LofMc
