It has been known for a long time that, among non-passerine birds, the Anatidae are remarkable for the diversity of specializations in the trachea and syrinx, associated with sound production. It has also been recognised that these variations have taxonomic significance (Heinroth 1911; Johnsgard 1961), but virtually no attempts have been made to correlate the complex tracheal structures with the sounds produced. Yet the group offers many fascinating problems such as the adaptive significance of the looping of the trachea, either outside the body cavity as in the Magpie Goose Anseranas semipalmata, or inside the sternum as in the northern swans Cygnus cygnus and C. columbianus. Additionally, males of many species of ducks, particularly pochards (Aythyini) and sea ducks (Mergini), exhibit irregular enlargements of the tracheal tube. The majority of anatine species also show marked sexual dimorphism in the structure of the syrinx. Asymetrically enlarged bullae are typical in adult males, while females retain the relatively simple condition similar to that found in geese and swans. This paper is a first and perhaps naive attempt to understand the functional significance of such complex variations in waterfowl vocalizations.
Available at: http://works.bepress.com/paul_johnsgard/39/