Meaning: This sentence describes the way birds sing at dawn. It's like saying,
Choice of Words: "Beaks" are the mouths of birds. "Tweet" is the sound made by birds. "Cheerful" means happy and optimistic. "Tunes" are melodies. "Dawn" is the first light of day. When you put it together, it means birds' mouths make happy and optimistic melodies at the first light of day.