My guess would be that bringing in new adult birds is more stressful. With chicks hatched (or even received as day olds) and raised in sight of the rest of the flock there is no immediate need to figure out the pecking order. The chicks are low not matter what and will slot in as they get older.
I think with adults brought in, they need to be physically separated by a fence of some sort so they can learn about each other. The existing flock will see them as competition for food and status.
I think with adults brought in, they need to be physically separated by a fence of some sort so they can learn about each other. The existing flock will see them as competition for food and status.