Strangers like me: birds respond equally to a familiar and an unfamiliar sentinel species' alarm calls, but respond less to non-core and non-sentinel's alarm calls

Alarm signals have evolved to communicate imminent threats to conspecifics but animals may also perceive other species' alarm displays to obtain adaptive information. In birds, mixed-species foraging flocks are often structured around a focal sentinel species, which produces reliable alarm calls that inform eavesdropping non-sentinel heterospecifics about predation risk. Ongoing work has revealed that several species can recognize the alarm calls of certain sentinel species even without prior encounters, including when these are from distant biogeographic regions. Similar work has yet to examine whether naive subjects' responses to unfamiliar sentinel alarm calls differ from responses to non-sentinel alarm calls. Here we played the alarm calls of three subtropical Asian bird species that participate in mixed species flocks, to temperate North American birds. Birds responded most to the alarm call of an allopatric core sentinel and a local sympatric sentinel control species, less so to an allopatric non-core sentinel, and least so to an allopatric non-sentinel and a negative control stimulus. These patterns provide evidence that broad phylogenetic and geographic recognition is a pertinent aspect of sentinel alarm calls in general.