Context-dependent vocal mimicry in a passerine bird

Abstract

How do birds select the sounds they mimic, and in what contexts do they use vocal mimicry? Some birds show a preference for mimicking other species' alarm notes, especially in situations when they appear to be alarmed. Yet no study has demonstrated that birds change the call types they mimic with changing contexts. We found that greater racket-tailed drongos (Dicrurus paradiseus) in the rainforest of Sri Lanka mimic the calls of predators and the alarm-associated calls of other species more often than would be expected from the frequency of these sounds in the acoustic environment. Drongos include this alarm-associated mimicry in their own alarm vocalizations, while incorporating other species' songs and contact calls in their own songs. Drongos show an additional level of context specificity by mimicking other species' ground predator-specific call types when mobbing. We suggest that drongos learn other species' calls and their contexts while interacting with these species in mixed flocks. The drongos' behaviour demonstrates that alarm-associated calls can have learned components, and that birds can learn the appropriate usage of calls that encode different types of information.

Figure 1

Spectrograms of the four categories of drongo notes: (a) mimicked alarm notes, (b) mimicked non-alarm notes, (c) species-specific alarm notes and (d) species-specific non-alarm notes. Uppercase letters represent the different types of mimicry recorded from at least three birds, as listed in table 1; a vocalization of the modelled species is followed by the imitation of a drongo. Lowercase letters designate the seven types of drongo alarm notes.

Figure 2

Drongos preferentially mimic alarm calls (open squares and solid regression line) to non-alarm calls (solid circles and dotted line) that occur in their acoustic environment (ANCOVA F_1,29=4.63, p<0.04).

Figure 3

The more drongos use their own alarm notes, the more likely they are to use mimicked alarm notes of other species, too. Of the 129 recordings that included both drongo species-specific notes and mimicked notes, 36 contained no alarm content (largest circle in lower left; the size of the bubbles reflects the number of data points). In contrast, in recordings in which at least 30% of species-specific notes were of alarm type, nearly all mimicry was of alarm type. This pattern was shown by all nine independent sets of recordings (Binomial test, expectation 0.5, p<0.002). Data rounded to the nearest 10%.