I don't have the picture anymore (sadly) but I once introduced days old chicks/a duck to weeks old chicks/one other duck. The ducks ditched the chicks and bonded with each other almost immediately, no surprise there.
What was more interesting was that the brooder looked separated by breed. I wasn't there often enough to say whether that was normal, but I had a picture where the youngest buff brahmas were following the oldest, and all the barred rocks were together, etc. It made sense to me because the young chicks seemed to think one of the older ones was their mom. Having a mom of the same pattern would add to their camouflage for predator protection.
Recently I also had a silkie who constantly wanted to brood. I let her sit on two eggs. One hatched, the other had movement but was behind. The hen got up with her active chick, leaving her remaining egg. Another hen went broody the day the first chick hatched (my hens form a queue for motherhood), so I gave her the egg to babysit. The new hen bonded with it and raised the single chick as her own.
I had already purchased a few chicks to give the first hatcher companions, just in case. But the chicks I bought and the first hatcher, all raised by the same silkie, never bonded. The two eggs who should have been raised together hung out behind their moms' backs more... I don't know if it was because they looked the same, or because they remembered each other from their egg days. The
TSC chicks, all different breeds, are still BFFs.
Anyway I just wonder how often that happens. I don't keep enough of the same breeds to know.