Translation:He wrote about the birds that lived in his garden.
The comment you quote seems to say that “the” is required because of “již,” not because the English sentence requires it. To me, “the birds…” means all the birds in his garden and “birds…” means just some of them.
So is there a Czech word instead of “již” that would convey the second meaning? Would using kteřy make a difference here?
kteří is possible here and does not change the meaning in any way
I don't get on your two meanings so I can't comment how to do them in Czech. It is just about the birds that live there, but it seems my English is not strong enough for such a distinction.
Alright, I guess I deserved that ☺! Of course it's my English that is by far not strong enough to say if “about birds” is a possible synonym of “about some (of the) birds.” It could be that birds of several species lived in his garden, and that he wrote about only one of them. But I fully agree it's not all that important. My bad that I brought it up in the first place. ☹