We've all heard the expression the pecking order, but to actually see it happen is something else.
When new chickens are introduced to an existing flock, within a couple of minutes the alarm will be raised and you will hear lots of agitated brk'ing noises from the incumbent chickens to alert all that "something is wrong". Soon the newcomers are identified and the peck-off starts. Literally the existing chickens peck (quite viciously) at the new chickens to remind them that they are above them. This process also gives everyone else in the flock a chance to move up (or down) a couple of pegs by pecking off a more senior chicken and perhaps get the mother-hen role !. This process can be a quick peck on soft skin such as the comb, ears, or a more aggressive pull and twist type action on the comb, feathers or similar.
This process can go on for 7-10 days and unfortunately the new chickens get a bit of a rough ride - unless they choose to stand up to their agressors. However everything sorts its self out and everyone will be best friends in a month or so's time. What is also odd is that those who were being pecked this time around are likely to be the worse offenders next time around and the more senior chickens delegate the "pecking duties" to the lower chickens, with an odd reminder from the senior chickens as to who is boss - by a quick reinforcing peck to the rest of the pack.
It is therefore important that chickens are not introduced in small (or odd) numbers, since a single new chicken would most likely get pecked to death as the sole newcomer, whereas if you introduce chickens in pairs or multiples of pairs, then they will pair up as life-long friends and it spreads out the pecking, hence simiplifying the introduction. Sometimes chickens will pair up to existing chickens (who look similar) which can leave one odd chicken to fend for themselves, these chickens may end up as loners and need to be monitored during introduction.
We use the "hosepipe of justice" in the first couple of days against the the chickens who are being over-efficient in their pecking duties. This works by giving them a short blast of cold water, because a cold, wet chicken decides its more important to preen than peck off the newcomers, however care is necessary to ensure that the chickens are not wet when its time to go to bed, particularly in cold weather as this will lead to the chicken not making it through the night. After a couple of squirts, they soon learn to stop and a squirt in the general direction breaks up any disputes.