I'm not saying these birds can't be happy. Some certainly aren't resocializeable, and there's nothing wrong with keeping them as happy as possible regardless.

One of the most objective criterion for animal welfare is how much of their natural behavior they can fulfill. People don't talk like birds, don't act like birds. Only parrots of pretty much the same species can fulfill some things. I'm not even talking about mating and all the behavior around that, more like everything else.