That would be "Kuka etsii lintua?" in the Finnish translation. Kuka becomes ketkä when there are multiple people.
Edit: Totally didn't realize "who are" isn't correct English... Thank you for pointing out. I'll still leave my comment in place in case someone is confused about "kuka" and "ketkä".
I agree. When 'who' is used as an interrogative pronoun (as opposed to its other function as a relative pronoun) it is wrong to make it the subject of a plural verb. The exception is a sentence such as 'Who are they?' or 'Who are the best players?', where 'who' is directly equated to a plural noun or pronoun using the verb 'to be'.
In those cases, the simplest interpretation is to say that it's the following noun that is the subject and "who" is the predicate complement. Also, we say (in modern English) "It was them!" but "Who are they?" further demonstrating that "they" is the subject of that sentence, not "who".
I concur. There are many kinds of questions that can start with, "Who are..." but this isn't one of them.
If I had to guess why, I'd say it's because the speaker has no reason to assume that the subject is plural -- that there is more than one entity searching -- since the identity of the subject is what is in question in the first place.
That's just a guess though. Ultimately, it comes down to a grammaticality judgment that "Who are looking for the bird?" sounds wrong to native speakers of English.
True. That's basically an echo question though, equivalent to other questions where the question word is not moved to the beginning of the sentence. "They said what?" The way those questions are formed is to leave everything as it is except use a question word for what wasn't heard (or believed).
There is no word for 'people' in the Finnish sentence. The English translation was originally given as "Who are looking for the bird?", but this was changed in reaction to the user discussion above, which pointed out that "Who are looking for the bird?" is ungrammatical in English - it should be "Who is looking for the bird?". Accordingly "the people" was added to the translation in order to make the English translation grammatically correct while also preserving the plurality of the verb.
There is no need for the equivalent of "the people" (ihmiset) in the Finnish sentence, because it is permissible for the plural interrogative pronoun "ketkä" to stand alone as the subject of a verb.