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I'm not saying don't give it to them, I'm just saying be aware that it does contain oxalates, and maybe err on the side of caution and don't give large quantities of the stuff to birds that really need their calcium (like anyone who's growing or laying).
Obviously the oxalate levels of kale are not high enough to cause acute poisoning in poultry (as evidenced by the number of people here who use it as feed with no noticeable problems), but oxalates do 1) bind with calcium, which 2) prevents the body from absorbing dietary calcium, and so 3) effectively lowers the calcium levels in the birds' diet. If that's not compensated for it can cause long term problems.
There's a lot of other stuff in kale, too, that is good for poultry -- just be mindful that it reduces the bioavailability of calcium.