Meaning: This sentence means that the small warm-blooded vertebrate animal that has feathers and lays eggs was feeling pleasure or contentment and produced musical sounds with its voice which were very attractive or pleasing to the eye.
Choice of Words: Little means small in size. Bird is a warm-blooded vertebrate animal that has feathers and lays eggs. Happily means feeling pleasure or contentment. Sang is the past tense of the verb sing, which means to produce musical sounds with your voice. Beautiful means very attractive or pleasing to the eye.