Warbling Vireo (Vireo gilvus)
Song: A variable series of warbling whistles, lasting 2,5 - 3.5 seconds, and following a distinctive undulating pattern. The first several notes tend to be low and soft, rapidly gaining in volume and following a step like pattern up and down the frequency range several times in the course of the song, often terminating at the top of the range and with a harsher, wide frequency band note.
Songs are delivered at a rate of about 7 per minute, the patterns being highly variable with very little repetition in note structure between songs.
Warbling Vireo song, July 11, 2010, Waukesha County, Wisconsin. This shows the typical undulating song pattern terminating with a wide frequency band note.
Howes-Jones, D. 1985. Relationships among song activity, context, and social behavior in the Warbling Vireo. Wilson Bull. 97:4-20.