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What do animal signals mean? 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. These studies have challenged the historical assumption that such referential signaling is a unique feature of human language and produced a paradigm shift in animal communication research. Over the past two decades, an increasing number of field studies have revealed similar complexity in anti-predator communication of birds. 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. 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. In this review, I explore the existing evidence for referential signaling in birds and highlight the importance of the cognitive approach to animal communication research. I hope this review will promote further investigations of alarm-calling behavior in birds and will help enhance our understanding of the ecology and evolution of semantic communication.

Abstract The selective pressure exerted by avian brood parasites forces their hosts to evolve specific defense strategies. When subject to brood parasite attack, avian hosts will often emit alarm calls. To date, few studies have examined whether and how host responses to different alarm calls indicative of different enemies vary with the host’s breeding stage. We carried out alarm call playback experiments during both the egg and nestling stages of the oriental reed warbler Acrocephalus orientalis, a host of the common cuckoo Cuculus canorus. The playback exemplars were selected from recorded alarm calls of the warbler to the presence of common cuckoos, sparrowhawks Accipiter nisus, and oriental turtle doves Streptopelia orientalis, which represented brood parasite, predator, and harmless control, respectively. The results showed that the oriental reed warblers did not discriminate alarm calls issued to different intruder types, but the intensity of the response was significantly higher in the nestling stage than in the egg stage. Attack behavior related to sparrowhawk alarm calls was absent in the egg stage, but aggressive behavior increased dramatically and exceeded the attack frequency in response to the cuckoo alarm call in the nestling stage, implying a shift in the tradeoff between the parents’ own survival and the loss of offspring. Alarm calls attracted a larger number of conspecifics than members of other species. In general, the oriental reed warbler had consistently stronger responses to different alarm calls in the nestling stage than in the egg stage, supporting the offspring value hypothesis.

The variation in the acoustic structure of alarm calls appears to convey information about the level of response urgency in some species, while in others it seems to denote the type of predator. While theoretical models and studies on species with functionally referential calls have emphasized that any animal signal considered to have an external referent also includes motivational content, to our knowledge, no empirical study has been able to show this. In this paper, I present an example of a graded alarm call system that combines referential information and also information on the level of urgency. Acoustically different alarm calls in the social mongoose Suricata suricatta are given in response to different predator types, but their call structure also varies depending on the level of urgency. Low urgency calls tend to be harmonic across all predator types, while high urgency calls are noisier. There was less evidence for consistency in the acoustic parameters assigned to particular predator types across different levels of urgency. This suggests that, while suricates convey information about the level of urgency along a general rule, the referential information about each category of predator type is not encoded in an obvious way.

Many mammalian and avian species produce conspicuous vocalizations upon encountering a predator, but vary their calling based on risk urgency and/or predator type. Calls falling into the latter category are termed \"functionally referential\" if they also elicit predatorappropriate reactions in listeners. Functionally referential alarm calling has been well documented in a number of Old World monkeys and lemurs, but evidence among Neotropical primates is limited. This study investigates the alarm call system of tufted capuchin monkeys (Cebus apella nigritus) by examining responses to predator and snake decoys encountered at various distances (reflecting differences in risk urgency). Observations in natural situations were conducted to determine if predator-associated calls were given in additional contexts. Results indicate the use of three call types. \"Barks\" are elicited exclusively by aerial threats, but the call most commonly given to terrestrial threats (the \"hiccup\") is given in nonpredatory contexts.The rate in which this latter call is produced reflects risk urgency. Playbacks of these two call types indicate that each elicits appropriate antipredator behaviors. The third call type, the \"peep,\" seems to be specific to terrestrial threats, but it is unknown if the call elicits predator-specific responses. \"Barks\" are thus functionally referential aerial predator calls, while \"hiccups\" are better seen as generalized disturbance calls which reflect risk urgency. Further evidence is needed to draw conclusions regarding the \"peep.\" These results add to the evidence that functionally referential aerial predator alarm calls are ubiquitous in primates, but that noncatarrhine primates use generalized disturbance calls in response to terrestrial threats.

A major evolutionary force driving functionally referential alarm calls is the need for different strategies to escape various predator types in complex structured habitats. In contrast, a single escape strategy appears to be sufficient in less‐structured open habitats, and under such conditions urgency‐dependent alarm calls may be favored. Nevertheless, some species, such as meerkats (Suricata suricatta), have evolved functionally referential alarm calls despite living in open areas, using only bolt‐holes for retreat. To understand the evolution of different alarm call systems, we investigated the calls of sympatric Cape ground squirrels (Xerus inauris) and compared their antipredator and foraging behavior with that of meerkats. Cape ground squirrels emitted urgency‐dependent alarm calls and responded to playbacks depending on urgency, not predator type. Vigilance behavior and habitat use differed between the two species. Meerkats roam widely to find prey and for efficient foraging depend on coordinated predator vigilance and escape behavior. As herbivores with smaller territories, Cape ground squirrels depend less on coordinated antipredator behavior, and urgency‐dependent alarm calls encode all essential information. We conclude that habitat complexity does not explain the evolution of functionally referential alarm calls in all species, and other constraints, such as the need to coordinate group movements to maintain foraging efficiency, could be more relevant.

Some mammals, including some nonhuman primates, use multiple play signals during social play that convey the playful mood or intention of the signaler. However, why single species use multiple play signals has not been investigated. We recently identified seven such signals among free-ranging juvenile rhesus monkeys (Macaca mulatta) on Cayo Santiago, Puerto Rico, all of which predict the imminent occurrence of play. Here we ask whether the use of these multiple signals may have more precise or multiple functions. Specifically, we ask whether different play signals are associated with different (1) types of play to come, (2) intensities of play, (3) initiators of play and/or (4) distances at which the signal is given. Our results indicate that most signals were disproportionately associated with one or more aspects of play. For example, gamboling was associated with play that is initiated at a distance, crouch-and-stares were associated with chasing play and intense play, and leg-peeks were associated with play initiated by the receiver, as opposed to the signaler. These nonrandom associations suggest that play signals, most of which are conspicuous body signals, are not merely redundant variants of a single general function to transmit a playful mood. Rather they may be used more selectively in a variety of play contexts in which it may be beneficial to reinforce, clarify or emphasize the playful intention of the signaler. At the same time, most are not associated with particular contexts in a highly exclusive or simple manner. As such further research is needed to determine whether they are examples of functionally referential signaling.

Field observations and acoustic analyses have shown that suricate (Suricata suricatta) alarm calls vary in their acoustic structure depending on predator type. In this study, we tested whether receivers respond appropriately when hearing a call in the absence of a predator. Although the only way for suricates to escape from predators is to retreat to boltholes, responses to playbacks could be divided into distinct categories. The subjects responded differently to alarm calls given in response to aerial or terrestrial predators and to recruitment calls emitted in response to snakes and deposits on the ground. Suricates also showed rather distinct responses to low, medium and high urgency aerial calls. Differences in the responses were less obvious for different levels of urgency in the terrestrial and recruitment calls. Suricate receivers thus gain information about both the predator type and level of urgency from the acoustic structures of their calls.

Functionally referential alarm calls have stimulus specificity, distinct acoustic structure, and elicit different escape responses that are appropriate to the threat. The mechanisms by which escape responses are evoked are not fully understood and may range from eliciting innate responses to conveying representational information. White-tailed ptarmigan ( Lagopus leucurus ) are a long-lived alpine tundra grouse, which are preyed upon by aerial and terrestrial predators. We investigated the hypothesis that alarm calls of ptarmigan hens with chicks are functionally referential. We recorded hens’ alarm calls in response to naturally occurring and model predators in California’s Sierra Nevada alpine tundra for two summer seasons. We conducted playback experiments in the field to determine chick responses to alarm calls. Alarm calls commenced with an extended ‘alerting’ note followed by a series of staccato notes grouped into elements. Fundamental and dominant frequencies in element notes were significantly higher in terrestrial compared to aerial threat alarm calls. Playbacks of terrestrial threat alarm calls elicited an upright/alert position by chicks (75 % of responses). In response to aerial threat alarm call playbacks, chicks flattened to the ground and froze (80 % of responses). To our knowledge, this study provides the first empirical evidence of functionally referential alarm calling, including the responses of the receivers, in an avian species in the wild.

Flight-song displays usually include distinctive vocalizations and behaviors that are probably energetically expensive and also likely to attract the attention of aerial predators. We observed flight songs performed by Dusky Flycatchers (Empidonax oberholseri) in two breeding populations. During 15 yr at Tioga Pass, California, we observed the display twice and, during 9 yr at Steamboat Mountain, British Columbia, we observed 21 flightsong displays. These displays were generally observed later in the breeding season, suggesting that they were not used for courtship, mate attraction, or territory defense. However, for 83% of these displays, a raptor was observed to be present, usually either an American Kestrel (Falco sparverius) or a Sharp-shinned Hawk (Accipiter striatus). We hypothesize that these displays, like stotting or other pursuit-deterrent signals, may be directed at small raptors that represent a greater threat to Dusky Flycatchers and might deter attack by signaling awareness. However, because only males were observed performing flight-song displays, it is also possible that these displays serve to alert mates and offspring about the presence of an aerial predator and reduce their vulnerability to attack. La conducta de cantar mientras se vuela incluye vocalizaciones distintivas y conductas que probablemente son energéticamente costosas y que pudieran atraer la atención de depredadores. Observamos cantos al volar en dos poblaciones del papamoscas Empidonax oberholseri. Durante 15 años de estudios en Tioga Pass, California y nueve anos en Steamboat Mountain, Columbia Británica, observamos la conducta previamente descrita observamos la conducta en dos y 21 ocasiones, respectivamente. Los cantos a vuelo fueron observados tardes en la época reproductiva, lo que sugiere que no fueron utilizados para el cortejo, atraer la atención de parejas prospectivas o defender el territorio. Sin embargo, en el 83% de las ocasiones, se observaron también a individuos de Falco sparverius o de Accipiter striatus. Proponemos como hipótesis, que la conducta observada, al igual que otras conductas de protección, pueden ser dirigidas a rapaces pequeños que representan una amenaza a los papamoscas y que pudieran detener el ataque indicando precaución. Sin embargo, dado el caso de que solo se observaron machos del papamoscas exhibiendo la conducta, es probable además que la conducta sirva para alertar a las parejas y pichones de los vocalizadores sobre la presencia de un depredador y de esta manera reducir su vulnerabilidad de ataque.