From the book

“And the female also sings! Her song repertoire is much like his . . . When she duets with her mate . . . she almost always matches the particular song that he is using.”

The female sings first in this sequence, with four down-slurred whistles. After that, listen for the longer songs of the male in the background, the louder female in the foreground, as she answers him with the same song that he is singing.