Maternal vocal control of behavioral inhibition in mallard ducklings (Anas platyrhynchos).

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Abstract
The different responses of mallard ducklings to 2 types of maternal vocalization uttered by the hen while brooding the young, the assembly call and the reconnaissance (or alarm) call was studied. The 2 calls differ with regard to repetition rate, frequency modulation, dominant frequency and note duration. Field observations of 6 hens with their broods revealed that the vocal behavior of ducklings is maintained by the assembly call and inhibited by the reconnaissance call. Since the 2 types of calls are most clearly separable from one another on the basis of repetition rate, laboratory experiments were conducted to assess the extent to which vocal and locomotor behavior of maternally naive (i.e., incubator-hatched) domestic mallard ducklings are differentially affected by fast (3.7 notes/s, typical of assembly calls) and slow (1.4 notes/s, typical of reconnaissance calls) repetition rates. Experiments using normal and altered-rate assembly and reconnaissance calls revealed that slow rates (normal reconnaissance and slowed assembly calls) inhibit all duckling vocalizations, and prompt the ducklings to freeze, whereas faster rates (normal assembly and quickened reconnaissance calls) excite the ducklings'' vocal behavior, and prompt them to approach the sound source. Repetition rate, which was the most important acoustic feature underlying the attractiveness of the mallard assembly call, may play an important role in vocal inhibition and freezing responses of ducklings.