Because it is the plural of "un bel oiseau" and that French keeps an article in the plural form, while the English doesn't.
Why are both "de" and "des" correct? I thought I had to use "de" beause the adjective precedes the noun.
Duolingo doesn't know that...
Another correct solution:<pre>
Des beaux oiseaux</pre>
I'm still confused about why "les" can't be used here. I realize a literal translation would make it "the beautiful birds", but I keep seeing "la/le/les" used for a translation without an article in English. I saw "beautiful birds" had no article in English so I tried using "les" for the French translation.
Consider this: "beautiful birds" is the plural of "a beautiful bird".
In French: "de beaux oiseaux" is the plural of "un bel oiseau".
Note: in front of an adjective, plural indefinite article "des" becomes "de" Note2: "beau" becomes "bel" in front of a noun starting with a vowel (oiseau) or a non-aspired H (homme)