Care needs to be taken when introducing new pullets (or grown hens) to your flock. Chickens take their pecking order very seriously and any new addition is seen as a threat to their place in the order. Even your sweet, affectionate hens might literally peck chicks or pullets to death if they are added suddenly.
Formerly sweet hens all of a sudden become 'mean girls', prowling the run, like grade school bullies in a school yard, looking for unsuspecting little ones to peck and put in their place.
Any time new members are added to (or taken away from) a flock, there will be a disruption in the pecking order. Some picking and squabbling should be expected but not to the extent that any real damage is done or you see blood.
In a perfect world, new chickens/pullets would not be added until they are about the same size as the others, never before they have been separated within in the run but behind fencing for at least two weeks - or quarantined AWAY from the others for at least 30 days (VERY important in the case of pullets or adult birds acquired elsewhere being added to your flock for biosecurity reasons) - and at least two birds (and preferably more than two) at a time.
That said, it's not a perfect world and things don't always go as planned. I personally don't have the set-up to house chicks separately from day-old to 6 months or more waiting for them to be full grown so they can join the others in the run. But I have developed an 'integration program' that seems to work well.
My chicks start out in a plastic storage tote complete with a little dust bath and roosting bars. See more about baby chick care here.
They soon outgrow the tote and at about two-three weeks old or so they move into this brooder cage in our garage, still under a heat lamp.
As soon as the daytime outside temperatures are the same as temperature in the brooder cage, I bring them outside during the day. Depending on the time of year I get my chicks, that can be anywhere between 4 and 8 weeks old.
I have a small pen that I put in the run so the little ones are safe but can get used to being outdoors AND the big girls can get used to the newcomers. The pen clips to the side of the run so I can move it around into the sun or shade as needed.
You can do something as simple as sectioning off a part of the run with chicken wire to keep the grown hens (or ducks) out.
I make sure they have shade, as well as protection from the wind, and plenty of feed and water. As you can see, there is a lot of curiosity among the grown hens.
During this time, I collect them all and bring them back into the house to sleep each night,because the night time temperatures are generally still too cold for them to spend the night outside. They tends to be the most tedious part of raising chicks, I have found. I use a cat carrier to tote them back and forth.
If you live in a warm climate, depending on the time of year, a small rabbit hutch/attached run inside the regular run would work quite well as transitional 'housing', allowing the pullets to sleep outside also. Just be sure that all openings are less than 1" or snakes, rats and mice can get in.
After about ten days of having the little ones outside like this during the day where they are safe from the older hens' beaks, the curiosity tends to wear off and the eventual face-to-face go far smoother.
The benign curiosity on the part of the older chickens is obvious in the short video below. But don't let your sweet hens fool you - curiosity can turn to nasty behavior in the blink of an eye.
When you are finally ready to let the little ones out into the run with the general population, be sure that you distract everyone with some treats or a pile of leaves or grass. It helps if there are places the little ones can run and hide to get away, such as under branches or even under a board placed on bricks or cinder blocks. I find that branches that the little ones can perch on also help get them away from the older hens, who seem to only want to chase those on the ground.
Don't spray the newcomers with vinegar or rub them with dryer sheets (don't laugh, I have read both things on other blogs!). You don't need to add them to the coop after dark, although sometimes after a day in their pen, I will just pop the pullets into the nesting boxes to sleep instead of carrying them back to the house. That way they are warm, safe, and as long as I get back down to open up the coop just before daybreak, they won't get picked on.
Watch the two groups after introducing them to each other. If you notice one hen being excessively mean, removing her from the run for a couple of days to a dog crate can do wonders for her disposition. It will knock her out of her place in the pecking order. Once she returns to the flock, she will be more concerned with regaining her place in the pecking order than terrorizing the newcomers.
As for feed, if you have layers and non-laying pullets in the run together, put everyone on starter/grower. Pullets shouldn't eat layer feed because it has too much calcium and it can damage their kidneys.
The laying hens will be fine on the starter/grower for a few weeks, but be sure to provide oyster shell (or crushed eggshell) free-choice in a separate dish for the laying hens - and check it often - because they will eat a fair amount to get the calcium they need for their egg shells.
You will want to set up a few 'feeding stations' around your run so a few bullies can't prevent the little ones from getting to the food and water.
It's a good idea to have some Blu-Kote on hand in case the pecking gets too bad. Blu-Kote is an antiseptic spray that masks any red raw spots or blood in purple. Chickens are attracted to the color red, so the Blu-Kote helps prevent further pecking.
An injured pullet should be treated immediately to prevent further pecking, but resist removing her immediately from the run. Removing her will just put her back at the bottom of the pecking order when she returns. It is far more productive to remove the bully (or bullies) if possible. If you aren't sure who they are, then remove the injured chicken along with several others the same age, so when you re-add them all back after a few days, she won't be the lone 'newbie'.
If the pecking and squabbling seem relentless, block off part of the run with chicken wire and try keeping the two groups separate for a few more days and then try it again. Eventually they do work it out and peace will reign again in your run. At least until the next time !