Lack of response to conspecific juvenile vocalizations in two avian brood parasites

Juveniles of interspecific avian brood parasites need to locate and recognize conspecifics to continue their life cycle after being reared by individuals of another species. However, little is known about the recognition mechanisms and cues involved in this critical stage of their lives. It has been proposed that adult parasites could show some kind of parental behavior by actively searching and interacting with conspecific young, which in turn could serve the juveniles to learn and/or reinforce the learning of conspecific characteristics. Since acoustic communication is one of the main channels used by birds, if such form of parental behavior by adult avian brood parasites does exist, adult cowbirds should recognize and respond positively to vocalizations of conspecific juveniles. We experimentally tested whether adult shiny Molothrus bonariensis and screaming M. rufoaxillaris cowbirds respond positively towards acoustic signals of conspecific juveniles using two playback experiments, one conducted in semi-captivity and the other in the field. In both cases, we presented 3 types of playbacks to adults of both cowbird species: vocalizations of shiny cowbird juveniles, vocalizations of screaming cowbird juveniles, and white noise as a control. In the experiment in semi-captivity, no significant differences were found between treatments. In the field experiment, no conspecifics approached the playback area, but hosts of the species whose playback was played were attracted to the loudspeaker. In conclusion, our results do not support the predictions we tested from the parental behavior hypothesis and indicate that adult shiny and screaming cowbirds do not respond positively to acoustic signals of conspecific juveniles, at least not to the type of signals used in our experiments.