This question has made my day! :D
It could have such a meaning, but then we'd use a comma, not a dash.
Is Duolingo recognising punctuation now? I typed "ducks are birds" and got it right, but comment appeared: "Another correct solution: ducks are birds." That's exactly what I typed. The only discernible difference between what I typed and the other solution is the full stop (the period).
I'm wondering the same thing as it threw me off. I would think the sentence would read "утки - птицы" if the intended translation was "Ducks are birds." The addition of зто to me adds a "these are" to the sentence, but my answer, "Ducks - they are birds", was incorrect.
«Это» is not unneccessary (without it, the sentence would sound really unnatural), but it is not translated «those», it's just a way we form definition-like sentences in Russian.
«Those birds are ducks» would be «Э́ти пти́цы — [это] у́тки». In this version, «э́то» would be optional. I can't explain well why it's optional.
«Утки — это птицы» and «Утки — птицы» mean the same thing in Russian, but the former is much more natural than the latter.
I do not understand why Это is used here. In previous examples, это is used for a demonstrative purpose, like: "Это лтицы" which would mean "These are birds" Therefore, would this sentence be translated as: "Ducks - these are birds" and of course the dash equates to nouns. So, why is the statement not simply: "Утки - птицы"?
It says the answer ducks're birds is a typo. 're literally means are, my answer is correct.
Duo just assumed it was a typo. If there was some way for it to be sure you intended to write ducks're, it would have marked your answer incorrect because it was clearly not a correct answer.
There is no such contraction in written English. People sometimes slur ducks and are into one word when speaking but there is no such. It is just sloppy speech.