...in practice, you aren't going to feed your birds enough cruciferous veggies to reach those levels, even if you somehow do, they aren't going to eat them given other choices. AND if you have a dish of oyster shell out for them, they will correct for it naturally.Spinach isn't really high protein. Its mostly water. Same with kale. and while there is some concern that high levels of oxalates present in all cruciferous veggies can affect calcium absorption, in theory...
So if it makes you happy, go ahead, give 'em spinach. kale. chard. broccoli. arugula. collards. bok choy. cabbage. Unless there's a bug on a leaf, mine ignore it.
Rabbits??? Whole 'nother story.