Anti-Predation Responses to Conspecific versus Heterospecific Alarm Calls by the Nestlings of Two Sympatric Birds

SIMPLE SUMMARY: Birds can adjust their behavior by reacting to sound cues. Parents can adjust the feeding rate of nestlings by their response to begging calls. Acoustics play an important role in parent–offspring communication. When adults perceive predators, they can make mobbing alarm calls to warn other individuals. Chicks can also respond to the alarm calls of parents and thus reduce their predation risk. We tested whether the chicks of two species could hear, and respond to, conspecific and heterospecific acoustic alarm signals. Chicks were found to eavesdrop on conspecific and heterospecific mobbing alarm calls. Acoustical similarities between the alarm calls may explain why chicks can recognize heterospecific calls. ABSTRACT: Predation is generally the main cause of bird mortality. Birds can use acoustic signals to increase their predation survival. Bird response to mobbing alarm calls is a form of anti-predation behavior. We used a playback technique and acoustic analysis to study the function of mobbing alarm calls in the parent–offspring communication of two sympatric birds, the vinous throated parrotbill (Sinosuthora webbianus) and oriental reed warbler (Acrocephalus orientalis). The chicks of these two species responded to conspecific and heterospecific mobbing alarm calls by suppressing their begging behavior. The mobbing alarm calls in these two species were similar. Mobbing alarm calls play an important role in parent–offspring communication, and chicks can eavesdrop on heterospecific alarm calls to increase their own survival. Eavesdropping behavior and the similarity of alarm call acoustics suggest that the evolution of alarm calls is conservative and favors sympatric birds that have coevolved to use the same calls to reduce predation risk.