As mentioned in the title I'm looking for a specific manga. I believe it was a oneshot but can't be certain. The main characters are an author and his young charge, whose age I'm unsure of. The author appeared to be in his twenties, while she appears to be in her mid-teens. Basically, they live together and there's some kind of sexual tension between them with the author being dominant or aggressive in a teasing manner. It has a serious tone but there's no blatant display that he's stepped over the line and is abusing her.

At some point, another male character, who I believe is the author's editor comes to his house and meets the pair. There seems to be some kind of attraction between the editor and the girl, which makes the author jealous. The editor becomes suspicious of their relationship. Near the end the girl reveals that she's not as naive as the author and the reader/audience had assumed. She is in fact very satisfied with her relationship with the author and returns his love.

The final pages involve the editor protesting to the author. The former mentions he feels like the latter is stifling the girl, but the girl tells him she's happy with how things are.

I believe the girl equated herself to being a bird in a cage and mentioned that such a thing didn't necessarily mean the captive was unhappy. I think the bird in the cage motif may have been used in the title but am not sure.

Honestly, I was worried the story was going to be tragic or something, so I was surprised by the turn around from the girl's reveal.

Sounds like it could be Wild Rose's Crown, the oneshot in Conflict Lover.
(Reading it again just now I'm less confident in my answer, but...)

~4400 manga completed
~1700 ongoing...

Yes. This was the manga I was thinking of; I just didn't remember some of the details clearly. Thank you.