Being placed into a new flock can be quite stressful for a chicken.
The other hens are often aggressive towards a newcomer. This is why it’s best to introduce new chickens in pairs - this way the attention won’t all be focused on one bird alone.
Making new friends – hens get to meet, beak to beak
Another tip is to introduce the newcomers in the evening, when the other flock members are already settling down for the night. This will give your new chickens a chance to get used to the new living area and have a relatively peaceful first night in their new home.
Don’t put the chicken directly into the midst of the group, but keep them slightly separated from the others, at least for the first few days. This will help the other chickens get used to the new additions.
It helps if you can get chickens of similar age and size to the rest of the flock (although understandably you might want to bring new, point-of-lay birds to an ageing flock). Small or young chickens are more vulnerable to being harassed by other, larger and older, members of the flock.
Again, keeping some degree of separation will help. You should be on hand during those first meetings, just to make sure the aggression doesn’t escalate. A certain amount of fluster and pecking is okay - just make sure the new birds are not getting hurt. Eventually the ‘pecking order’ will sort itself out and the birds will settle into their new flock.