Asymmetric eavesdropping between common mynas and red-vented bulbuls

Abstract

Heterospecific eavesdropping on alarm calls is well documented, but less is known about the factors influencing asymmetry in the reliability of heterospecific alarm calls. Partial overlap of predators between heterospecifics has been hypothesized as 1 possible mechanism driving asymmetric eavesdropping. We tested the responses of common mynas (Acridotheres tristis) and red-vented bulbuls (Pycnonotus cafer) to reciprocal playbacks of alarm and social calls by measuring changes from baseline in the rates of fly-bys near the speaker and in rates of singing. We found an asymmetric communication network between bulbuls and mynas: bulbuls only responded to conspecific alarm calls, whereas mynas responded to both bulbul and conspecific alarm calls. This communication asymmetry may be due to a partial overlap in predators between species. Mynas were observed to spend time in both trees and on the ground and may be susceptible to both aerial and ground predators. We observed bulbuls primarily in trees and therefore may be susceptible primarily to aerial predators. If this is the case, then the alarm calls of mynas are less reliable to bulbuls compared with the reliability of alarm calls of bulbuls to mynas. However, further studies into the predators of each species are necessary before drawing a definitive conclusion. Our study demonstrates a differential responsiveness of 1 species on the alarm calls from another species for predator information and underscores the importance in considering heterospecific communication networks in the removal of species from a community.