Since Russian has dropped the verb 'to be' in Present case in most cases (although you can find it in formal documents), the sentences like «X is Y» became ambiguous. «X — Y» may be mistaken for an apposition «X-Y» (e.g. «курица-птица» might theoretically mean 'a chicken bird' or 'a bird of a hen'; in this sentence it's not a viable option, but in many sentences using «X — Y» would be abmiguous).
So, the Russian languare has resolved ambiguity by inserting это 'it'. While it originally was a means to reduce ambiguity, now sentences with это actually sound more natural.
Compare it with Mandarin Chinese: in Classical Chinese, 是 used to mean 'this', but now it is a verb 'is'. Unlike Mandarin, Russian hasn't gone so far as to make «это» a verb, but this sentence still sounds better with «это».
While English, since it has never really dropped 'is', doesn't usually use 'it' in such situations, so that's why we leave «это» out in translation.
In this sentence construction " - это" means "like that". In conversation we insert "это" to speak clearly.
For example "Курица, птица" and "курица - птица" sound exactly the same, but these have different meanings. That's because all punctuation marks sound like a pause.
So we mostly say "курица и птица" when we see 2 birds (exept "курица, птица")
We say "курица - это птица" when we speak about chickens (exept "курица - птица")
The hyphen means hidden word or hidden some words. In this situation hidden word is "является" (to be in English).
In addition to the singular vs. plural thing, Russian doesn't have articles. Basically everytime there's a noun, it could be either "the" or "a/an" in English. So in this case a straightforward translation is "(a/the) chicken is (a/the) bird" and other translations are dubious/technically complicated.
But... I would also be interested in whether the sentence can have both the literal and the abstract meaning.
I agree with you, but for the purposes of learning Russian, the learner has to know the difference between singular and plural. These exercises shouldn't be loose translations as long as the meaning is understood but should be as precise translations as possible in order to show the learner's aptitude.
If duoLingo tells me that "Chickens are birds." is incorrect, then I'll believe that there is a problem with my understanding of "Курица - это птица." and won't be sure if its a generalized statement because "A chicken is a bird." and "Chickens are birds." are the generalized statements in English.
Yes, nuance is everything. But there is precious little difference between "A chicken is a bird." and "Chickens are bird.". I would have to say they're the same thing. They both mean that chickens belong to the bird family or the class aves. The question is does "Курица - это птица" mean that chickens belong to the bird family? The child asks "Why do chickens lay eggs?" The adult answers "Because a chicken is a bird" or "Because chickens are birds." Same thing.