Have the chicks ever been in a brooder where there has been hardware cloth? How cold is there brooder? Is it outside or inside the house? I have not seen chicks with cracked or black beaks before. Something they are pecking or touching is causing the damage. The difference in the size is because the big ones are 3 weeks older. The ISA browns do not look like meat birds. I would keep a close eye on them to see how the beaks are getting damaged. Sharp metal feeders could be the problem, but hard ro know for sure.
Before I forget, the australorp chick in the second photo, may possibly have splay legs, but it could be the slick floor. Keep their brooder bedding non-slip, and look at this link about splay leg treatment:
https://www.chickenheavenonearth.co...obbling-chickens-chicken-heaven-on-earth.html
Before I forget, the australorp chick in the second photo, may possibly have splay legs, but it could be the slick floor. Keep their brooder bedding non-slip, and look at this link about splay leg treatment:
https://www.chickenheavenonearth.co...obbling-chickens-chicken-heaven-on-earth.html