I didn't think about this possibility! You are right: if the bird is eating the duck, then Vogel is the nominative object (the subject) and Ente is the accusative object (the direct object); in this case, "Eine Ente isst ein Vogel." means the same thing as "Ein Vogel isst eine Ente.". (I checked this with a German teacher to be sure).
I forgot that, in German, the terms in the sentence can be reordered in such manner (ironically, in Portuguese, my native language, this is also possible). Thank you for having observed that!
Tatiane, German is more flexible than Portuguese when it comes to ordering the terms in the sentence. This happens because of its declination system, which marks the syntactic function of a noun independently of where it is placed in the sentence structure.
We can reorder terms in Portuguese sentences as well, but we rely greatly on semantic features, rather than syntactic ones, to maintain the meaning and avoid ambiguities. For example, you can change "O poeta escrevia um doce verso" into "Um doce verso escrevia o poeta", but you can't invert "A menina beijou o menino" into "O menino beijou a menina" without changing the meaning.
This happens because we don't change the articles ("o", "a", "os", "as") according to the role the following noun plays in the sentence. However, German does that, which allows us to make inversions that are impossible to be made in Portuguese without altering the meaning.
In this case, if you want to say "Um pássaro come um pato", "pássaro" is the nominative object and you should use "ein", while "pato" is the accusative object, and you should use "eine". In this case, the final German sentence can be either "Ein Vogel isst eine Ente" or "Eine Ente isst ein Vogel" and, because of the declinations, both sentences mean the same thing.
On the other hand, if you want to say "Um pato come um pássaro", than "pato" (the nominative object now) should be preceded by "eine" and "pássaro" (the accusative object now) should be preceded by "einen". Again, the final sentence can be either "Eine Ente isst einen Vogel" or "Einen Vogel isst eine Ente".
Because when you use the verb sein, what follows it is in the nominative case. This is because sein is a linking verb, not an active verb (I don't know the proper German terms, but this is the idea). In this case, Vogel is a predicate noun (http://www.vistawide.com/german/grammar/german_cases_nominative.htm).
Is it possible for Duolingo to actually publish a dictionary / word list with nouns that include gender? I keep getting questions wrong because it is the first time I am hearing or seeing a noun and its gender has not been introduced. Serious failing of Duolingo is the lack of a dictionary that includes gender articles
Even further, I'd love if they would give you the opportunity to practice vocab using articles with the nouns so you can memorize their gender. I know you could do it separately, but it'd be much easier and more fun to practice your learned/introduced vocab here instead of having to look them up in the dictionary and then use a separate app like Quizlet to study.
How strange that no-one has yet mentioned the woman's pronunciation of 'Vogel'. I've listened to it slowly several times - it still sounds like she's saying 'Vollmer', or something like that. I was wondering why Duolingo would suddenly insert a new, unfamiliar word. By contrast, the man speaking the same sentence clearly says 'Vogel'.
Wait a minute! This sentence is translated as, "A duck is a bird." There is no "isst" in the sentence. The correct word is "ist." A duck is a bird. It is not eating a bird. I am not sure that the sentence is true, but that is what it says. It is "ein Vogel" and not "einen Vogel" because with the verb "to be" it is always a predicate in the nominative case.
You have to be really careful to write "Ente" instead of "Schwanz"...
The verb "to be", in all languages, never has an object: The duck does actually nothing to the bird, it is the same as the bird.
"Ein Vogel" is a predicate of the subject, and as such, is in nominative, not accusative; so masculine singular accusative indefinite article is "ein".
Similar verbs are "werden" (become), "bleiben" (remain)…
The grammatical gender has really not much to do with the thing; only with the word.
The word "Vogel" is masculine, so it is "der Vogel", whether it is a female or male bird.
On the other hand, the word "Ente" is feminine, whether it is a male or female duck.