In Shelducks, as in all Anatidae species, duckling calls are uniform in both sexes. However, adults show great sexual dimorphism in vocalizations. The objective of this study is to discover how differences in male and female repertoire develop. Are duckllng vocalisations used for metamorphosis or are new calls introduced? 29 ducklings were hand-reared in different groups, and additional data from wild and captive families was also sampled. Sound recordings were analysed by sonograph. Duckling repertoire consists of 5 call types in both sexes. In females call pitch is rising until day 6, then continually falls to a minimum around day 70. The amount of noise structures in the calls increases remarkably from day 40 onwards ('voice break'). Duckling element structures partly remain recognizable. Adult calls are fully developed at 80 days of age. Female repertoire consists of low-pitched rolling or quacking sounds. ' In males call pitch rises and falls as in females. Some noise structures appear but voice break is hidden behind a 'mute phase'. In the course of further development juvenile type elements reappear. New notes are inserted between them to form the 'tyu-tyu' call. At 5 months of age, 2 different types of trills are created by high speed repetition of 'tyu' calls. Adult male repertoire is fully developed at 8 months of age and consists entirely of pure tonal whistles. To summarise: Female calls are developed straight from juvenile repertoire by lowering of pitch and an increasing amount of noise structures. Male call development first follows the female path. Later new elements appear and new call types (trills) are created. Male calls undergo a longer lasting secondary development.
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