A chicken is. a chicken is a chicken right or a I missing something
Yep,
Well you might think so but. Different breeds and even different birds within a breed have different personalities. Too, different lines of the same breed can be different in size, hardiness and utility.
I got three Dominiques. One was a runt, one got sick and died and one Was much larger, went broody and was a very protective mother. They were very bossy in that they'd be the first in the coop a night and squawk til the others came in. Not all hens or hens within a breed will brood eggs. Some make lousy mothers.
Just like people in a family they can be different. Some are friendly and some are not. Some like to forage and some aren't so good at it.
Some breeds are more hyper and others don't like to be confined. Buckeye's were developed for winter hardiness. I have friend in Israel who couldn't raise Brahmas cuz it was to hot.
I have Birchen Marans roosters father and son. Both raised from chicks, but the son has attacked me more than once. I now carry a stick when I have to go to his coop. His father nice as can be. Some hens lay dark eggs other spotted.
Hybrids while better layers, can be more susceptible to disease and problems. Lay lots of eggs in the first couple years and then drop off. Purebreds can lay the same amount of eggs but over a longer period time.
This is why I suggested "Storey's Illustrated Guide to Poultry Breeds." It not only covers chickens but Geese, Turkey's and Ducks. Even then you'll find the exception to the rule.