Using playback to test leadership in mixed-species flocks and compare flocking with mobbing

Playback can help identify keystone species in communities that interact behaviourally. Specifically, playback has been used to identify leader species in mixed-species flocks (MSFs) of birds, although it is not clear whether responding heterospecifics are attracted to the leader species or the whole MSF. Playback can also simulate a mobbing response; both flocking and mobbing are regarded as adaptations to predation, but the species participating in these phenomena are not often compared. We performed a complex experiment, comparing heterospecific responses to playback of (1) MSFs (multiple species calling simultaneously), (2) a primary leader species, (3, 4) two secondary leader species that occasionally lead MSFs, (5, 6) two follower species, (7) one nonflocking species and (8) one predator (a small owlet). In 240 trials over 2 years, 72 species made 980 heterospecific approaches to the speaker. We found the predator treatment attracted the most species, and the weakest response was to the nonflocking species. Other patterns were unexpected, however: the response to the MSF, and to the primary leader, was not higher than to other treatments that included MSF members, and the response to one follower species (the particularly vigilant grey-headed canary-flycatcher, Culicicapa ceylonensis) was higher than that to one secondary leader. Although flocking propensity did not influence the response, species that followed MSFs responded more than those that led them. There was a strong correlation between which bird species responded to the owlet and which responded to any MSF member. Our results reveal (1) a response by flock followers to all MSF participants, as if they used these species’ vocalizations to search for the whole MSF, (2) some weak evidence that vigilant species are particularly attractive, and (3) a general correspondence between the species that are attracted to an MSF and those that mob.