I have heard different things on giving cabbage, I have never personally given it but I have read alot of places that say cabbage is ok.
Feed only small quantities but vary the offerings made.
Choose from the following list:
carrots - cut into thin, beak-size strips
silverbeet - include the white spine
whole string beans
corn on the cob
celery - stalks and tops
This is what was posted on a national cockatiel society.
I also found this on a vet hospital site
fresh varying foods should be offered as well.Vegies esp. greens like spinach silver beet endive parsley carrots peas celery parsley and cabbage are great.