Older chickens pecking on younger chicken

so sorry to hear of your loss .

i am not chicken wishper but  here is a few thing to look into frist if you have a heat lamp was it a  clear one if so i would go to a red one ASAP! this will calm them down . then i am sure they had plently of feed an water, they get board so i would get a flock block from tractor supply about $12.00. get some brocclie hang from a string so they an peck at it also carrots cabbage thing like that.

check the list of things not to give chickens on this site very informational .

also do you have plenty of room in the coop they get a little crazy if not enough room  i use about 5sq fet per bird if not more.

 also what feed do you use some times if the protein is to low they will get like that. i use KENT e-xtra egg and mulitiflock i like it.

 have you check for mites or lice that can cause some stressed out birds.

 as for worming i am sure that would not be a factor but some thing i would get done once they are clam agian wait a while.  i have 118 birds and i have some chicks of different ages it is some time diffcuiate to intergrade them all to gether in the winter, i wait till they are all about the same size i dont have to much problem then. some times there is just that one who thinks they need to beat on every one so we have a nice little pen always set up for that chicken time out as we call it after a day or more  they  get the message are glad to be back in the flock

  i hope this helps and you dont have that problem agian. if it just one bird that is the culpert you might need to cull it

i had a leghorn that  just kept  picking on one of my younger chicks she would find any way of getting to her.she  put a hole in her head so needless to say she is no more but after a few stitches  cinder the chick is well not sure what happen but i will not keep a bird with a bad attuide. she did not look sick some times they just pick on the weak but i saw not sign of that . i think we just  got  one mean leghorn

  good luck !! and dont get discourage it will get better.learnig about them is half the adventure!

Bigger chickens pick on smaller chickens, that is just the way it goes in chicken society.  Why this didn't start upon introduction - I have no idea.  Perhaps they have been recently cooped up due to bad weather and boredom set in.  Pecking/cannibalism is a difficult habit to break once it has been initiated.  Do not reintroduce the younger birds until they are mature.