Chickens are tribal birds. As such, they stick together with the birds that look like them, or the ones they have familial connections with. In over a decade, many breeds (mostly consisting of heavy production breeds) have gone through my coops, and the behavior I've witnessed is the same. Each breed has formed close bonds with the birds they resemble the most. The leghorns stick with the leghorns, the ISA brown with the ISA brown, and so on. I had a single black bird, at one point. She was picked on constantly, and was driven almost to the edge of their territory. Same thing happened with the Blue hazes I periodically got. More recent examples, at one point, there was only one normally feathered female, and a pair of silkied bantams, who always stuck together. The silkied male passed, and after a few months I introduced a normally feathered male. The loner became the silkied female, even though she'd gotten to know the second male for longer. In my only batch of Tsouloufates, the male came out more stocky. Apart from the young pullets that follow him, he's gained the following of the most stocky adult female, who had been loyal to his father for almost two years. His father is not as stocky as him. In addition, the hen doesn't have a crest. The cockerel has a small crest, while his father has a bigger one. Slight differences like these are enough to cause group splits. One does not notice them, if one does not free range, since the splits are not visible, but they are present. I had an ISA brown get sick almost three years ago. She got picked on by everyone, but her ISA brown sister. And I can assure you, there is nothing more relentless than an ISA brown hen (except for actual gamefowl). It matters to me that in case a bird is poorly, I will not have a case of extreme aggression and shunning caused by the breed difference. That sick ISA brown was never able to be re-accepted back into the flock, since the only bird who accepted her after she was treated, was her ISA brown sister. I'm positive
@Shadrach has had similar experiences, and
@fuzzi has a great photo of their speckled Sussex bantams roosting at the other edge of the perch from their Dominique bantams (if I'm not mistaken on breeds). Of course, if one has a flock consisting of completely unrelated birds, with every bird being a different breed, or looking nothing alike, then it does become too mismatched to group off, but that is another thing entirely, and something I do not have experience with
