Assessing "false" alarm calls by a drongo (Dicrurus paradiseus) in mixed-species bird flocks

The suggestion that some members of mixed-species bird flocks use alarm calls when predators are not present in order to startle other species and thereby gain access to additional prey, first postulated by Munn (Munn CA. 1986. Birds that 'cry wolf'. Nature. 391:143--145.), has generated considerable interest due to its implication that the calling birds are intentionally deceiving listeners. Despite this interest, "false alarms" have been studied rarely and without detailed acoustical analysis. We explored whether false alarms are used by Greater Racket-tailed Drongos, which produce a distinctive set of notes when alarmed. We found that drongos did indeed make false alarms, defined as an alarm vocalization made either while the bird was on the perch or in the air, and followed within 10 s by a foraging attempt. However, the acoustic features of the call notes used, particularly of those calls made in the air, were more similar to aggressive calls, made when drongos chased each other, than to actual alarms produced when predators were present. Drongo foraging success was greater after false alarm calls than after silence or nonalarm vocalizations, and playback of false alarms in the air induced escape behavior in other species, though at a lower level than actual alarms. Thus, although drongos can use false alarms to startle other birds and gain foraging opportunities, such calls cannot be called "false" with certainty because they may also signal aggressive intent. Indeed, aggression and alarm may be intertwined in this family of birds as drongos actively mob or chase many predators, including bird-eating hawks. Copyright 2010, Oxford University Press.