the opening of the spring concerto in Vivaldi's four seasons

Listen to the mockingbird, where it's become traditional to whistle bird-like sounds along with the chorus

There's a tune in O'Neill's Music of Ireland called the Goldfinch. One day I decided to play it on the flute in such a way as to make it sound more like a bird. For example, a bird sings no octaves, and its timing is not regular.

Try something like that yourself, it's fun.