Birds often live in environments with multiple types of risks such as predators (Caro ).
To survive predation hazards that vary based on predator attributes, birds have evolved a sophisticated communication system using alarm calls that may show discrete variations (different call types), graded variations (number of sound elements such as note number and calling rate, or finer acoustic features of an element such as call length, frequency/pitch, and relative amplitude), and combinatorial variations (combination of notes or calls).
These observations have been anecdotal for a long time, as no experimental studies have been conducted to test whether senders vocally discriminate between those different threats and how receivers respond to those calls.
Over the past two decades, an increasing number of studies have applied the experimental approaches established by Evans () to examine the information content of alarm calls in wild birds.
However, the classical framework of functional reference has led researchers to neglect these acoustic variations and the pragmatic aspects of animal communication (Scarantino and Clay ), there have been only a few systematic investigations of referential communication in other animals such as birds.
Thus, we are still far from a comprehensive understanding of how extensively functionally referential signals are involved in animal communication systems and which selective pressures drive the evolution of semantic communication in animals.The goal of this review is to condense previous studies on the semantics of avian alarm calls from a wide spectrum and advance our understanding about the ecological significance, cognitive processes, and evolution of semantic communication.) examined the vocal responses of male chickens to two types of predators (flying raptors and raccoons) through a video monitor and found that they produce acoustically discrete types of alarm calls for these two predatory stimuli.The researchers further revealed that playbacks of these calls elicit discrete types of anti-predator responses in hens (Evans et al. However, in natural situations, fowls produce aerial alarm calls in response to a variety of non-threatening stimuli, such as flying insects and harmless birds, and often mix those two types of alarm calls in an anti-predator context (Gyger et al. The acoustic structures of aerial and mobbing alarm calls are quite different within a species and might therefore be used to communicate information about the behavior and/or spatial position of the predator.These variations in alarm calls are typically associated with the predator’s attributes, such as predator type and distance, and receivers respond to them with appropriate anti-predator behaviors.Although alarm calls of several bird species, as well as those of monkeys, appear to denote predator attributes, almost nothing is known about the cognitive processes that underlie the production and perception of these signals. This is a central question in studies on animal communication.Research into the semantics of animal signals began in 1980, with evidence that alarm calls of a non-human primate designated predators as external referents.To test whether the variation in certain calls is linked to perception specificity, researchers have conducted playback experiments in the absence of actual stimuli (i.e., predators or food) (Evans ).If these calls contained sufficient information about the external entities, receivers were expected to exhibit appropriate responses to the playback of calls as if the eliciting stimuli (e.g., predators or food) were nearby.Frameworks for the study of semantic communication about external objects or events.The functionally referential framework a only considers the association between production specificity and perception specificity as information transmission, whereas the cognitive framework b assumes mental representations of external entities in information processing ), these animals often produce different alarm calls for a variety of predators, and some of them can transmit many different types of information to receivers.