It is a soft liquid yodeling, which the male bird uses to call the scattered flock together.
"Ways of Wood Folk"
William J. Long
"It is a good day, a lucky day," said the Potato Face Blind Man, "because for a beginning I have heard high in the elm trees the flummywisters yodeling their yisters in the long branches of the lingering leaves.
"Rootabaga Stories"
Carl Sandburg
It came from behind high walls and hung among the leaves of the trees, a phantom yodeling, the welcoming "you-you-you-you" of the women of Islam.
"O. Henry Memorial Award Prize Stories of 1921"
Various