Many animals recognize the alarm calls produced by other species, but the amount of information they glean from these eavesdropped signals is unknown. We previously showed that black-capped chickadees (Poecile atricapillus) have a sophisticated alarm call system in which they encode complex information about the size and risk of potential predators in variations of a single type of mobbing alarm call. Here we show experimentally that red-breasted nuthatches (Sitta canadensis) respond appropriately to subtle variations of these heterospecific "chick-a-dee" alarm calls, thereby evidencing that they have gained important information about potential predators in their environment. This study demonstrates a previously unsuspected level of discrimination in intertaxon eavesdropping.
