I'm a firm believer that they need to learn how to be birds. There's a lot to being a bird - the preening alone is a big deal & they don't seem to be totally "hardwired" as to how to do it correctly. I think pulling them too early results in a lot of the problems we later see, such as plucking. I suspect a lot of that could be avoided if the birds had a good solid "base" both emotionally and in terms of learned behavior, about how to be "proper" birds.
I think also it's like breastfeeding a human baby - there are lots of long term health benefits to breastfeeding that we are continuing to learn about (more than just immediate immune system boosting, although it certainly does that too). I think it has to work the same way physically in birds - there are bound to be many benefits from letting the parents feed them.
I think co-parenting is best. I'm not a breeder though, so I don't know what the time line would be like for starting them on more feedings & increasing interaction with humans. Probably different according to the species of bird, I'd guess. I also think it's a good idea to let them fledge, for a variety of reasons, mostly to build confidence & teach them how to properly use their wings. Birds who understand what their wings are actually for seem less likely to mutilate them, whereas many severely plucked birds have destroyed their wings.
