Weird slowly progressing fatal (respiratory?) disease

[AudieWarren]

@rebrascora, I didn't even consider Marek as the symptoms didn't seem to fit, but if you say they may vary, I won't exclude it.

But if there will be a necropsy, I'll most likely have to do it myself. What should I look for?
Local vet services are not very helpful when it comes to chickens.

Where are you located? I'll help you look for a place that does necropsy. Mine was done by the state and not a vet. Vets here don't tend to chicken either, but the commercial chicken industry is booming.

 

[Kiawaki]

@rebrascora, I'm in Croatia. A veterinary university sounds like a good option, thanks for the idea.

In the end, as you say, I'll probably have to rely on the survivors as a base for a more resistant flock. It's just so frustrating when you have 80% or more of the young birds die, and I was hoping to get some rare breeds this coming spring.