Strong asymmetric interspecific aggression between two sympatric New Guinean robins

Interspecific aggression is predicted to be common in diverse tropical avifaunas but is seldom tested experimentally. I conducted two playback experiments to measure interspecific aggression between two sympatric Australo‐Papuan robin species that inhabit the understorey of montane New Guinean forests: Slaty Robin Peneothello cyanus and Ashy Robin Heteromyias albispecularis . The first experiment, at a location where both species were present, demonstrated strong asymmetric interspecific aggression; the larger Ashy Robin responded as aggressively to Slaty Robin playback as to conspecific playback, but Slaty Robins did not respond to Ashy Robin playback. In contrast, in the second experiment, which took place at a nearby site where Slaty Robins were absent, Ashy Robins did not respond aggressively to Slaty Robin playback. Thus, Ashy Robin interspecific aggression appears to be a learned response to the presence of Slaty Robins, and may reflect resource competition between these two sympatric species.