A better way of wording it would be that the bird is in the accusative case because it is the object that the verb is being performed on, thus it recieves the -n suffix. Adjectives have to agree with the noun they are attributed towards, which is why bela also gets the -n suffix.
Wouldn't "lovely" be "ama"? I'm not totally sure, because I haven't progressed too far in the course, but you can make nouns adjectives by changing the "o" at the end of a word to "a". So amo, meaning "love", can be changed to "ama", or lovely.
Mi vidas belan birdon: I see a beautiful bird. Mi vidas aman birdon: I see a lovely bird.
(Take this with a grain of salt, though--can someone comment if I'm wrong?)