I heard That the Hung Munchin/Bird is CG

That would be a trick since it was made decades before CG was possible.

If you watch the scene the bird is visible the whole scene and you can see it spread it's wings. You can see a peacock as well when they revive the tin man.

No, I read up on that story. The myth goes that if you look in the background during the part where Dorothy, the Scarecrow, and the Tin Man head off on their adventures (this is right after we meet the Tin Man, and he decides to go with them), up in the trees, you'll see a man hanging himself. The myth goes that a set worker committed suicide and they accidentally filmed him hanging there.

However, the real story is, one way the film-makers wanted Oz to look more otherworldly was to show tropical birds living there. Unfortunately, the birds that were brought in got loose, and you can see them on set in some of the shots. You can see a toucan up in the trees when the Wicked Witch is watching for Dorothy and the Scarecrow to approach. The other was a crane. (There were several other birds mentioned in the story, but I can only remember the toucan and the crane).

Now if you look in that same shot mentioned earlier, you'll see something brown and skinny moving among the apple trees. That was the loose crane, not a man hanging himself.