"The bird is eating the flies."
Translation:Der Vogel frisst die Fliegen.
The hint ordering got confused because the word "the" shows up twice in the sentence.
I believe that in such a case, it always sorts the hints the same way.
For the first "the" (in "the bird"), der is an appropriate hint.
For the second one (in "the flies"), it's not.
But the hint ordering isn't clever enough to know which "the" corresponds to der and which one corresponds to die and it shows the same ordering for both.
Which is one big reason why you can't rely on the hints for "recommendations" or "suggestions".
The words are Vogel, Bär, Katze with a capital letter at the beginning -- all German nouns are capitalised; the capitalisation is part of their spelling. There is no German word vogel or bär or katze.
We say der Vogel because the word Vogel is masculine.
We say die Katze because the word Katze is feminine.
We say das Pferd because the word Pferd is neuter.
The grammatical gender of a noun in German is mostly arbitrary. Just look it up and remember it.
(Also, der Katze would be appropriate in the genitive or dative cases, as der is the article not only for masculine nominative but also for feminine genitive, feminine dative, and plural genitive.)
why isn't it: "Der Vogel frisst den Fliegen"?
The flies are the direct object of the verb fressen, so they have to be in the accusative case.
den could be masculine accusative (wrong gender -- Fliegen is plural, not masculine) or plural dative (wrong case -- we need accusative, not dative).
Duo said the correct answer was "Der Vogel isst die Fliegen"... incorrect.
Why do you think it's incorrect? Which part do you think is incorrect? What do you think it should have been instead?
Pointing out that something is "incorrect" does not help anybody else unless you say why it's incorrect and what it should be instead.
Also, talking about "the correct answer" (as if there were only one correct answer) is nearly never useful -- there are nearly always many accepted translations. Though only one or a small handful are considered the "best" translations.