Meaning: A small bird is making a beautiful sound with its voice.
Choice of Words: Little describes the bird's size, bird is the animal, sings is the action it takes, sweet describes the song, and song is what the bird produces.
Alternative Expressions
The little bird is singing a sweet song.
Meaning: This also means the bird is making a beautiful sound with its voice.
Choice of Words: This sentence is similar to the first one, but it adds the word the before little bird to make it more specific. Is is used to connect the subject little bird to the verb singing.
The little bird's song is sweet.
Meaning: This means the bird is making a beautiful sound with its voice.
Choice of Words: This sentence is slightly different from the first two. It uses the possessive form bird's to show that the song belongs to the bird. Is is still used to connect the subject bird's song to the adjective sweet.