A tiny amount of plain, unsweetened yogurt is tolerated by most parrots despite their lactose intolerance, but it's not something to offer regularly.
Keeper's noteIf you use it at all, plain and unsweetened only, in dabs — handy for hiding medication. Flavored or sweetened yogurt should be skipped entirely.
What to watch out for with yogurt
Birds are lactose intolerant, but yogurt's live cultures pre-digest much of the lactose, so a small amount of plain yogurt usually causes less upset than milk would. It's still dairy with no real dietary need, and flavored yogurts add sugar. There's no strong benefit, so it stays an occasional, tiny treat.
How to serve it
Plain, unsweetened only; a small dab occasionally.
Never flavored, fruit-on-the-bottom, or sweetened yogurt.