I've got a nominative versus accusative question about this sentence. I have a confusion about this in English as well.
'Ptáci' is clearly nominative. That particular 'kluk' pattern shifts the spelling for each of the nominative/accusative plural/singular combinations.
The pattern shifting is not so clear with 'kachny,' 'slepice' and 'husy.'
Here's my question: I was surprised the this sentence called for 'pták' in the nominative. I expected accusative. If'pták' is nominative, what case are 'kachny,' 'slepice' and 'husy'? Accusative, right? 'Pták' or 'birds' are subjects of the sentence in both languages? And the species are the objects?
All are the nominative case in this sentence. They are all essentially subjects, because the verb "are" connects them. The primary difference is that pták is masculine animate, while the other three are feminine. The secondary difference is that kachna and husa follow a different declension pattern than slepice. You can find the pattern for kachna at this very useful site, and do a search for the others there as well: https://en.wiktionary.org/wiki/kachna. (Click Declension if the table isn't visible.)
In "X = Y" sentences, both X and Y are in the nominative case. There's no need for other cases in simple "Something is something" sentences. (Although it's possible to use instrumental, if you want to express that the state is only temporary, which clearly doesn't apply to the sentence in this exercise.)