Hello Christian. This is going to require a really in depth answer so if I can get some more information from you, it will help me give you my best advice and guidance.
When did this baby hatch?
Are there other eggs as well?
Is the baby still with both parent birds? If not, why not? Has something gone wrong?
Is the baby in a nest box or where?
Are both parents tending it?
Please tell me everything in the parents diet because we need to make sure they are getting everything they need to stay healthy during this process.
Is this your first experience having a bird hatch for you?
Do you know anything about hand feeding baby chicks.?
The more information I have, the better I can help.