YOU SAID:

the bird flew so high we couldn't see it

INTO JAPANESE

鳥はとても高く飛んだので見えませんでした

BACK INTO ENGLISH

The bird flew so high that I could not see it.

INTO JAPANESE

その鳥はとても高く飛んだので見えませんでした。

BACK INTO ENGLISH

The bird flew so high that I could not see it.