Sorry but you have this completely back to front.

There's no such thing as "too rich in calcium", I mean we feed turts pure calcium as a supplement and you can't get richer than that!

Kale is one of the few Brassicas that you can feed safely, cabbage is one of the worst and you should stop feeding that, it's high in oxalates which prevent the conversion of calcium into bone and shell, same with spinach.

Kale on the other hand is high in calcium but relatively low in oxalates.