The acoustic structures of avian alarm calls show a high degree of variation in pitch, duration, shape, and repetition rate.
In addition to such distinct and graded variations, several birds combine discrete types of notes or calls into higher complex sequences.
Despite the increasing number of studies that demonstrated evidence for functionally referential signals in animals, there remain unanswered questions and controversy about the semantics of animal signals (Rendall et al. First, although functionally referential signals seem to convey information about the environmental entities of the senders beyond their internal states, in most cases, it remains controversial whether such communication can be considered an evolutionary precursor of linguistic reference (Rendall et al. Even when a sender produces different calls for different stimuli and receivers respond to them with qualitatively dissimilar reactions, such behavioral cascades could occur simply as a result of associative learning without any construction of mental representations (Wheeler and Fischer ), and no known study has investigated representational ideation in senders.
Second, it may be problematic that previous studies bias researchers to consider a call as functionally referential only when it is acoustically distinctive from other call types.
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.Recent theoretical work indicates that, even when the signal structure is not highly specific to external objects, receivers may still be able to derive “referential” information from the signals by virtue of contextual cues (i.e., pragmatics, Scott-Phillips ).Therefore, it may be possible that graded and combinatorial variations in acoustic structure may also contribute to the transmission of information about external entities and could be considered functionally referential.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).First, I summarize the relationship between acoustic variation and information content of alarm calls.Then, I explore which signal variations could be used to refer to external entities and how researchers can investigate cognitive processes that underlie signal perception and information processing.In consideration of these three issues, I explore the evidence for communication about external entities in wild birds.Over the past two decades, a number of field studies have been conducted to investigate the information content of avian alarm calls.