Heterospecific attraction to the vocalizations of birds in mass-fruiting trees

Abstract

A mixed-species aggregation refers to a group of individuals of different species attracted together by an environmental factor. Although they are traditionally considered not as structured as moving mixed-species groups, the composition and organization of aggregations can be influenced both by competition and by information flow between species. Frugivorous birds, for example, might move towards vocalizations of other species as cues to resources. If they differentially respond to a combination of other species’ vocalizations, an indication of a generalist fruiting tree, this could be a mechanism that creates nested structure in seed dispersal networks. We used both observational and experimental methods to study a community of frugivores in southwest China. Although the foraging niches of the birds highly overlapped, little interspecific aggression was found, suggesting a superabundant food resource. Birds were more attracted to playback of mass-fruiting trees, recorded when they were fruiting, than recordings made when those same trees were not fruiting. We compared attraction to the vocalizations of (a) an insectivorous species, (b, c, d) three species of frugivores, each presented separately, and (e) a combination of the frugivores in which their vocalizations were repeated sequentially. Response of heterospecific frugivores to the combination tape was significantly higher than to the insectivorous control, and tended (P = 0.068) to be higher than response to the frugivores. We suggest that this represents an indication that birds can use the diversity of vocalizations in a tree as a cue to understand the community of trees they interact with, as well as the community of birds. To our knowledge, although now used frequently with mixed-species bird flocks, playback has not been used to explore whether frugivorous birds could use heterospecific vocalizations to find fruit. We present the results of a simple, realistic experiment that shows that respondents approach trees in which birds, the majority of which are heterospecifics, are vocalizing. The results from a more complicated, controlled experiment suggest that birds may preferentially be attracted to a combination of frugivores’ sounds. We argue these results have relevance to the structure of seed dispersal networks, although we cannot demonstrate that specialist birds hone in on generalist trees by cuing on heterospecific vocalizations because of the rarity of specialists in our system.