Dynamics of heterospecific eavesdropping in ant-following birds of the Neotropics
Animals use public information (PI) to assess habitat quality, availability of mates, and the location of food and predators. Use of PI therefore has important consequences for individual fitness and social interactions among individuals. Animals obtain PI by eavesdropping on heterospecific or conspecific signals intended for another receiver. In the Neotropics, numerous bird species attend army-ant (Eciton burchellii) swarms and feed on terrestrial arthropods that are flushed while attempting to avoid the swarm. Attending birds often vocalize to maintain their foraging area at the swarm; both conspecifics and heterospecifics may use this PI to locate swarms and assess swarm quality. To test this hypothesis, we used a factorial playback experiment to simulate birds attending ant swarms and examined behavioral responses to playbacks. We examined three PI factors (# birds present at the simulated swarm, # species present, level of dependence on ants of attending birds) that are hypothesized to be used by birds to assess swarm quality. We predicted that responses intensity would increase with: 1) increasing # individuals, 2) increasing # species, and 3) increasing level of dependence. We used point-counts to continuously estimate movement patterns, space use and vocal behavior of birds responding to playbacks.
We found that all three PI factors were important predictors of responses to playbacks, with level of dependence explaining the most variation in response, followed by number of species and then number of individuals. Responses by conspecifics to playbacks were not significantly different across treatments. In contrast, we found that the mean number of both heterospecific individuals and species that responded to playbacks increased with increasing group size, species diversity, and level of dependence on swarms. In addition, heterospecific species responded more quickly, approached closer, and spent more time at playbacks with higher group size, species diversity and level of dependence. Our results suggest that 1) ant-following birds use vocalizations of other birds attending ant swarms to locate the swarms 2) heterospecific eavesdropping appears to be a common mechanism for recruitment of birds to swarms and 3) birds use different types of PI to assess swarm quality. This is one of the first experiments to explore the dynamics of recruitment to ant-swarms in ant-following birds and demonstrates how birds use PI to influence complex behavioral decisions.