This is another one of these questions where when Duolingo poses it as a select a word question that there is more than one answer which might work. Please note that in the select a word question, we cannot see from what the sentence is based against until after we select something and it then gets revealed.
There were 3 choices:
- La birdoj flugis suben.
- La birdoj flugis supren.
- La birdoj flugis hejmen.
All of those above are perfectly good Esperanto. That means it's an utter crapshoot as to which one is the correct choice when doing a select a word, because we can't rule any out by virtue of being wrongly formed Esperanto.
I don't know how this occurs, but I've see it in two other lessons and it's really awful because it's not instructive in any meaningful way.
As it turned out, after I chose hejmen, the sentence that was hidden from me was "The birds flew upwards."
Whenever I get something like this, I report it and explain what happened. I think the course creators can disable certain types of exercises for a sentence, but they might not know that this particular sentence is acting like this in this exercise.
There are definitely some cases where this exercise is helpful in Esperanto, such as if the exercise had instead been something like "[Lia/Liaj/Lian/Liajn] birdoj flugis supren." It's more a result of this course being in beta and needing improvement that this exercise is bad, rather than this type of exercise being always bad for Esperanto. I'm sending in a report with a link to this discussion, just to be sure the course creators saw this and are aware of the issue with this sentence/the exercise you had (I just had a basic translation one).
I think it's an unfortunate side effect of the automated parts of Duolingo. The select a word question is more appropriate in some languages than others, for example it seems to work quite well in a Slavic language like Ukrainian, forcing one to note the case, but I agree it's less appropriate for a language like Esperanto. Unfortunately I think that's something that would need Duolingo to fix, not the course contributors and mods.
Besides being used to indicate a direct object, the accusative ending -n is used to indicate quantities, measurements, prices, distances, duration etc.
I agree with Izabela ! Any of the three choices is plausible. The birds can fly under(sub), over(above) and upwards (Supren)! In my question the ward hejmen that she mentions was not even shown. Besides I did not know that birds have homes! If they have something they have nests!!