"– What does the owl say? – Hoot."
Translation:– Mitä pöllö sanoo? – Huhuu.
Mikä is in the nominative case. This would be used for the subject of a sentence. Like 'Mikä sanoo 'huhuu'? Pöllö (sanoo 'huhuu')', "What says 'hoot'? An owl (says 'hoot')."
Mitä is in the partitive case. This would be used for the object of a sentence, as in Duolingo's 'Mitä pöllö sanoo?' Huhuu, "What does the owl say? Hoot."
The obvious onomatopoeic English translation for "Huhuu" is "Twit-twoo" ... though I suppose the various options for the spelling would cause controversy. And the fact that when this is heard it's actually two owls ... one calling "Twit" and the other responding "Twoo". I suppose "hoot" is a safe bet in comparison, though I consider it a more a verb ... e.g. the owl hoots ... the owl doesn't SAY "hoot"; same as a man shouts ... a man doesn't SAY "shout".
Just do a Google or Google Image search ... I'm not making it up. It's a very common English 'translation' of owl calls. I guarantee if you asked a Brit what animal says "twit-twoo" they'll say "an owl". https://www.expressandstar.com/news/uk-news/2018/09/27/public-urged-to-listen-for-twit-twoo-amid-concern-over-tawny-owls-decline/