[ToC]

 

Paul A. Johnsgard, Waterfowl, University of Nebraska Press, 1968

DIAGRAM OF ANATOMICAL VARIATIONS IN THE SYRINXES OF REPRESENTATIVE MALE WATERFOWL

One or more species of each tribe are shown, organized according to probable evolutionary relationships.

Male whistling ducks have a bulla, or a symmetrical bony enlargement at the base of the trachea, the shape of which varies somewhat in the different species (Figure 5). Females have considerably simpler trachea, but it is apparently impossible to judge the sex of a bird by its call.