I'm in the same boat with a check half the size of the others. This happened last time I had chicks and that one lived 5 months, didn't mature like the rest, and she suddenly went downhill then died. The vet saw her when she took ill, gave her antibiotics just in case it was bacterial but she suspected it was failure to thrive. I tube fed the bird for a few days then she had a a seizure and died. Now I have a small one again, using the left over baby parrot mash the vet gave me for tube feeding, mixing that with chicken crumble and hand feeding that chick to supplement what she gets in the brooder. I'm hoping this time to catch it early and try to get the bird growing. Both tiny birds are/were orpingtons, different breeders. Both lavender. My other lavender this time is huge (boy??)
Anyway, if anyone has any tips for anything else I can do to Jumpstart this chick, let me know. It's behind in maturity and feather growth, all chicks hatched Janunary 31.