There's no need to give a healthy bird Vitamin C. Their bodies manufacture their own Vit C. Won't hurt them so if you like to give it, then do it.

My macaw pulled out every leg feather after he ate a wedge of orange. They grew back and the next orange made him do it again. I guessed it was the stickiness, but am not sure. Anyway, he didn't get any more of them after the second time.

 

Certain illnesses can be helped by natural Vitamin C so at some point a vet may suggest feeding citrus.

Annie

I have a senegal and I give hime oranges but only about one segment a week or so as it is acidic.  He didnt used to like them but eats them now as well as apple, grapes (only 1 every few days cause of the sugar content ) Trying to get him to eat banana too but not yet :)

Oranges contain much that is good, beyond vitamin C.  They grow  in areas of the world that could overlap with many of these birds' natural environments, so they are a very natural food to give them.

 

My Amazon LOVES oranges. I give him about 1/6 of an orange usually  twice a week.  When my Grey was young, he would eat them but only the white part under the skin.  I mentioned to the vet that I thought that was odd.  He said that white part (pith) is where lots of the good stuff is  Bio-flavanoids with antibiotic, anti-bacterial, and anti-cancer properties.

 

Orange nutrition info