Breakdown of Jeden Winter fliegen sie vom Brutgebiet in das Winterquartier.
in
in
sie
they
fliegen
to fly
vom
from the (von + dem) / of the (belonging to) / by the (agent in a passive clause)
Questions & Answers about Jeden Winter fliegen sie vom Brutgebiet in das Winterquartier.
Why is Winter in the accusative case (jeden Winter) if there is no preposition in front of it?
This is the adverbial accusative! Time expressions that answer the question "when?" or "how often?" without using a preposition take the accusative case. Since Winter is a masculine noun (der Winter), the phrase becomes jeden Winter.
Why do we use the dative for vom Brutgebiet but the accusative for in das Winterquartier? Don't both involve movement?
It depends entirely on the prepositions. The word von (inside the contraction vom) is a strict dative preposition, so it always forces the dative case regardless of the context. in, however, is a two-way preposition. Because the birds are traveling towards a destination, it triggers the accusative case here (das Winterquartier).
Can I contract in das to ins in this sentence?
Yes, absolutely! Saying ins Winterquartier is very natural and arguably more common in everyday speech. Writing it out as in das just makes the sentence sound a bit more formal or places a slight emphasis on the specific destination.
Why is Winterquartier singular in German when the English translation says "winter quarters"?
The German noun das Quartier (accommodation, lodging, or habitat) is usually singular when referring to a specific base or a collective area. English happens to prefer the plural "quarters" for this concept, but the German singular is standard here.