Northern cardinal
Chapter 3: Why and How Birds Sing
Subchapter: Not one, but two voice boxes
From page 40 in the book.
♫121. Three examples for each of four different songs excerpted from a long sequence by a male cardinal (A A A B B B C C C D D D). May 28, 2010. Grundy, Virginia. (1:41)
♫122. One example of each of those four song types (A B C D). (0:26)
♫123. One example of each of those four song types, slowed to half speed (A B C D). (0:43)
♫124. One example of each of those four song types, slowed to quarter speed (A B C D). To hear the two voice boxes, listen carefully to the first ten seconds, which contains three slurred whistles. The first whistle has an abrupt break separating the two voices, but in the next two the transition between the two voices is more subtle, but still audible. (1:21)