Alarm call responsivity of mallard ducklings: V. Age-related changes in repetition rate specificity and behavioral inhibition.

Twenty-four-hour-old mallard ducklings (Anas platyrhynchos) exhibit a high degree of behavioral freezing (i.e., vocal and locomotor inhibition) upon hearing the maternal alarm call, which the hen utters when potential predators are near the nest. In this study, we assessed age-related changes in alarm call responsivity between 12 and 72 hr after hatching. Experiment 1 revealed that, although a significant reduction in vocal activity occurred upon exposure to the alarm call at all ages tested (12, 24, 36, 48, 60, 72 hr), birds older than 48 hr exhibited lower levels of freezing than younger birds. Furthermore, 12-hr-old ducklings exhibited the greatest level of freezing among the younger-aged groups. In Experiment 2, 12-, and 72-hr-old ducklings were tested to variations of the alarm call that differed in repetition rate (0.2 to 2.6 notes/sec, in increments of 2/10 of a second) to ascertain whether there are age-related changes in perceptual specificity and whether ducklings exhibit perceptual sharpening. Although older ducklings exhibited a slight shift in perceptual specificity toward faster repetition rates, they froze significantly less than younger ducklings at all repetition rates. Therefore, there was no evidence of perceptual sharpening. These data suggest that alarm call responsivity may represent an adaptation that appears to be associated with the timing of the nest exodus.