Leg Problems

My uncle had 4 Banty chickens 1 rooster and 3 hens

an he had other chickens to but he lets the Bantys live out in the yard so there not in the pen

and this pass summer the 3 hens have set an hatch out about 3 batches of eggs each all around the yard ( i no it's alot of babies...lol )

but anyway like out of each batch of babies like 1 or 2 babies would start to have leg problems like there cripple or something and we didn't no what was wrong with them

some of them died an some grew out of it an when they got bigger
there legs were fine like it never happen


do you guys no what it could be?