Wilson, 2011 – Fowl communicate the size, speed and proximity of avian stimuli through graded structure in referential alarm calls

Many animals produce alarm calls that warn conspecifics about predators. In some species, alarm calls communicate continuous traits associated with a predator encounter, such as its level of threat. In other species, alarm calls communicate categorical traits, such as predator class (e.g avain versus terestial) and are consequently considered fuctionally referantial. In theory functionally referantial alarm calls can simulatneously communicate continuously distributed traits, though examples of such calls are rare. Such dual-funstion calls could be adaptive because they would enable receivers to tailor their responses to a specific predator class, as well as to more subtle characteristics of individual attacks. Here, we tested whether male fowl (Gallus gallus) communicate continuous variation in avian stimuli through graded structure in their functionality referntial aerial alarm calls. In their first experiment, we held male fowl in an indoor test cage and allowed them to view wild birds flying past a window. We recorded their alram calls compared the structure to the size, speed, and proximityof the eleciti stimuli.  Stimuli that appeared closer, larger, and faster elicited alarm calls that were shorter, louder, clearer, and lower in frequency. In the second experiment, we broadcast alarm calls to foraging females and compared their responses to the graded structural changes documented earlier. Females exhibited greater initial reponses and finished feeding later in response to louder alarm calls. Together, these results show that fowl communicate the size, speed, and proximity of avian stimuli through graded variation in their functionally referential aerial alarm calls.