I translated this as "The green birds are there again" which was marked as incorrect. I was wondering why "da" would only be translated as "here" in this case instead of "there."
I'm having a brain block! Why is it grünen Vögel?. Is Vogel a weak noun, or it it accusative for some reason?
AzumiOhara's link is good. When the definite article (die) isn't in the nominative form (that would be der here), the adjective ending changes to -en.
Here's a little memory helper to go with it, from jess1camar1e:
-Big 3 get an -e (der, die, das) der alte Mann, das kleine Kind, die schöne Frau
-Changin' gets -en (plural and case changes) den alten Mann (accusative), der schönen Frau (dative), die kleinen Kinder (plural)
-No 'the'? Adjective takes over (no 'der' word or just an 'ein') Kaltes Wetter gefällt mir nicht (das Wetter). Ein guter Mann ist schwer zu finden (der Mann).
Note changin' means the definite article differs -- has changed -- from its nominative singular form (der, die, das). So feminine accusative, for example, doesn't get an -en: "Ich sehe die schöne Katze" -- I see the beautiful cat.
Also see this discussion here: https://forum.duolingo.com/comment/556140
Thanks for passing on the tip. Only trouble is I have to think through it every time I want to use an adjective before a noun, and that make my speech boringly slow!
I believe it is grünen because the declension of the adjective for plural nouns in nominative case with die article is -en. You can see the explanation here: https://www.duolingo.com/skill/de/Colors/tips-and-notes