Fact and fiction come together in a magically imagined tale of the composer Anton Dvorák’s inspiration for “American Quartet.” Transfixed by the birdsong of the little Iowa river town where he and his family are spending the summer, Dvorák seeks out the beautiful song of the scarlet tanager. And as the tanager woos a mate and begins to build a nest, Dvorák writes the bird’s music down on everything from paper to shirtsleeves. At the end of one long summer evening, the scarlet tanager hears familiar music from the town’s schoolhouse and settles on a branch nearby to listen. Inside, Dvorák and his friends play the first rehearsal of “American Quartet,” inspired by the little songbird. Then, while one plays on, the other flies home.