Our experience is that some breeds get along better together than others. In a smaller space chickens also get testy w/ each other w/ older ones harassing newer birds. Even in our open backyard for hens to forage, the older ones will chase/harass the newer birds, nothing injurious, but it's chicken behaviour. More space is a real help w/ lots of hiding spots ~ chairs, box crates, pen perches, tree stumps or boughs, hanging chicken toys, mirrors (a real winner for distraction), ladders, milk crates, chicken swings, dog house, etc, might help the younger to get away from oldsters' abuse? Whenever a hen goes broody others will get jealous & pick on her. Some chickens are just plain bullies & we've had to rehome a couple in the past.
Tish a Cuckoo Marans upset the whole flock she just wouldn't stop ~ rehomed to a neighbor's layer flock.She didn't warm up to humans either. She didn't bully in the layer flock.
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Char a very tall heavy Cuckoo Breda Fowl was a bully toward the littler Silkies so she went to a friend's rescue farm where she apparently got along well w/ a dog.
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I hope something will work out for you... but a large space w/ distraction objects really helps if birds are confined.
