Meaning: This sentence describes the action of a bird producing a pleasant musical sound.
Choice of Words: The words in this sentence are chosen to convey the idea of a bird producing a pleasant sound. "Bird" refers to a warm-blooded vertebrate animal that typically has wings and feathers, "sang" is the past tense of the verb "sing," which means to produce musical sounds with one's voice, and "sweetly" describes the quality of the bird's song.