I believe that "это" is common in normal speech for "definition-like" sentences.
So "a duck is a hen", (for "any duck in general, no matter which"), works fine with that "это" there. But if it was "the duck" (or even "a duck" but referring to one specific duck in particular and not the abstract "any duck"), talking about a specific duck mentioned before for example, I believe putting это there would be less common.
This was my first guess also (although I used "chicken"). Because of the dash, i thought that 'это не курица' is the main clause, 'утка' is subordinate. Hence: "it's not a chicken, but a duck (instead)". Also, every time I have seen 'это' used before this, it has always expressed "this", "here" or "it" in some way. Either as in "this thing is something" or "it is something" etc.
Is "It is a duck, not a chicken" wrong?
ку́рица (kúrica) [ˈkurʲɪt͡sə] f anim (genitive ку́рицы, nominative plural ку́ры or ку́рицы, genitive plural кур or ку́риц) "chicken (bird); hen; chicken (flesh)": From Proto-Slavic *kurica, from *kurъ (“cock”, onomatopoeic) + *-ica (denominal suffix, forming feminine counterparts of masculine nouns).