Nominative (der) is used for the person or thing performing the action - the subject of the sentence. Accusative (den) is used for the direct object of the sentence, the thing being acted upon. So we have Der Hund (performing the action of bringing something) and den Vogel (the thing being brought). Note the den form of accusative only applies to masculine (der) nouns like der Vogel. If the two animals swapped places it would become Der Vogel bringt den Hund.
Der Vogel is Nominative and den Vogel is Accusative. I'm not sure how to explain this to you because I don't know much grammar but I'll try to help you understand. That is THE bird (der Vogel) - nominative He is bringing THE bird (den Vogel) - accusative He likes THE bird (den Vogel) - accusative
Why do they always give the word definition but always expect us to know a possible answer?! I had the word that meant “is bringing“. I put “the dog is bringing the bird“ but it said “possible answer: the dog receives the bird“! I wanted to get it completely right!! This makes me sick.