Drongos select sounds to mimic non-randomly and use these sounds with high context specificity. Drongos' selection of sounds to mimic is non-random in that alarm-associated notes are mimicked out-of-proportion to their frequency in the acoustic environment. Drongos' performance of mimicry is context-dependent in that they change the notes they select depending on the alarm context. In effect, drongos call like other species would do in a particular situation: they use the song and contact calls of other species in non-alarm contexts, alarm calls of other species in alarm contexts and the mobbing calls of other species when they mob a ground predator. Exceptions to this rule are the vocalizations of predators and nest predators, which drongos mimic in alarm contexts.
We believe that similar behaviour is likely to be found in a diversity of passerine birds. We know that other species of drongos practice the behaviour, as we have seen white-bellied drongos (D. caerulescens
) mimic cat ‘meow’ calls and the alarm calls of squirrels while performing mobbing behaviour. Such behaviour is described matter-of-factly in the older ornithological literature: Vernon (1973)
states that D. adsimilis
‘imitates when it is alarmed and often includes the alarm notes of other birds’, although he does not elaborate on how he judged the birds' level of alarm. The observations of Robinson (1974)
; Morton (1976)
; Greenlaw et al. (1998)
and Chu (2001)
suggest that the behaviour is found in oscine passerines throughout the world. The relationship between mimicry and the alarm context has not been clear in earlier studies, however, because observers have noted that some alarm-associated calls were incorporated into song (e.g. Remsen 1976
; Greenlaw et al. 1998
), and that some non-alarm mimicry occurred in distress situations (e.g. Chu 2001
). It is probable that in these species, as in drongos, the correlation between the alarm context and the mimicry of alarm-associated calls is far from perfect. The ‘noise’ in the data may be intrinsic to the behaviour, or may result from inadequate human knowledge about rapidly changing contexts.
The most parsimonious hypothesis for how drongos acquire their mimicked repertoire is that they learn it directly from the species they interact with in mixed flocks. Drongo fledglings are fed by their parents in mixed-species flocks, thus exposing the young birds to other species' vocalizations. Drongos may learn predator vocalizations from the response of the flock as a whole: e.g. when serpent eagles fly over a flock they sometimes vocalize, and several species may make alarm calls simultaneously. There is also some evidence that drongos can learn mimicry from other drongos. Birds in several sites sang the distinctive jingle call (see ) that was always combined with mimicked notes and stands out from the rest of the drongo species-typical repertoire in its predictable, repetitive acoustic structure, yet, it is not clearly derived from any other species' call, and hence is likely to be transferred from drongo to drongo. Even if young drongos initially acquire their mimicked sounds by copying adult drongos, however, the behaviour of the other species in flocks would continuously reinforce the proper usage of these sounds.
Drongo mimicry demonstrates that alarm-associated calls can have learned components. Calls have been traditionally assumed to be genetically encoded (Lanyon 1960
; Marler 2004
), and those calls that are known to be learned are usually flight or contact calls (e.g. Mundinger 1970
; Nowicki 1989
). Previous studies have shown that the usage of mobbing calls can be altered by experience: Curio et al. (1978)
demonstrated that a mobbing response could be transferred to a novel object through a form of imprinting. Presumably, the birds in Curio's experiment did not learn to produce the sounds associated with mobbing, but only learned the context the sounds should be associated with. In contrast, our data and those of Chu (2001)
clearly indicate that some species of birds can learn how to produce mobbing and distress calls as well as how to use them. Our study further shows that birds can learn the production and context of alarm calls are associated with imminent threats (Klump & Shalter 1984
). For example, we once saw a drongo utter the alarm call of an orange-billed babbler immediately upon being startled by the sudden flight of another bird. Intuitively, one might assume that calls used in such rare and urgent situations would be ‘hard-wired’. Yet drongo mimicry, as well as experiments that show chickens (Gallus gallus
) can suppress producing alarm calls in certain conditions (Evans et al. 1993
), indicates that even alarm calls can be labile as a result of experience.
Drongo mimicry also shows that birds can learn to use heterospecific calls in appropriate contexts under natural conditions. It has been widely shown that animals can learn to recognize the context of the vocalizations of other species and react in appropriate ways (e.g. Hauser 1988
; Zuberbuhler 2002
). But using heterospecific signals in a contextually appropriate way has been previously shown only in artificial conditions by parrots (Pepperberg 1998
) and marine mammals (Ralls et al. 1985
). To understand why some bird species learn to produce heterospecific vocalizations whereas most species produce only conspecific sounds (e.g. Marler & Peters 1977
), and why a subset of mimicking species are able to use heterospecific signals contextually, will require further work on the function of mimicry in the field.
More generally, drongo mimicry demonstrates that birds are able to learn the appropriate contexts to an array of call types. Contextual learning has previously been demonstrated for songs, as birds are able to adapt their song repertoires in ways to match other individuals and even can learn the correct usage of song types that are used in different times of the day and for different audiences (Kroodsma 1988
; Spector et al. 1989
). But calls are different from songs in that they provide much more direct information about environmental conditions (Marler 2004
). Although we cannot conclude drongo mimicry is used as a label for an environmental stimulus (and is thus ‘referential’, in sensu Evans et al. 1993
) because mimicry could also reflect the animal's sense of risk (Blumstein 1999
), mimicked calls do provide reliable information to receivers. The ability to imitate calls contextually would allow drongos to use a novel signal associated with a particular environmental condition to communicate information about the environment to conspecifics and possibly heterospecifics. But such learning also has potential costs, because the correct usage of a call is an essential part of the signal, and improper learning of the context would thus jeopardize effective communication.