Neighbor Call Amplitude Influences Aggressive Behavior and Intermale Spacing in Choruses of the Pacific Treefrog (Hyla regilla)

Male Pacific treefrogs (Hyla regilla) aggregate in choruses during the breeding season. Within these choruses frogs distribute themselves nonrandomly. This study tested the hypothesis that the amplitude of neighbors' calls serves as a proximate cue in regulating the spacing of males in choruses, and that this is mediated by the incidence of aggressive-encounter calls by resident males. The amplitude of neighbors' calls showed little interindividual variation. Advertisement calls were played to males at three amplitudes that spanned the range of neighbor-call amplitudes measured between pairs of frogs. At playback amplitudes corresponding to the minimum neighbor-call amplitudes observed in choruses, frogs gave predominantly advertisement calls and few aggressive-encounter calls in response. As the playback amplitude was increased, subjects progressively decreased the number of advertisement calls and increased the number of encounter calls that they produced in response. The total number of calls (advertisement + encounter) given in response did not vary with playback amplitude. Intruders were likely to move away when a resident male gave encounter calls. In this way neighbor-call amplitude regulates intermale spacing in choruses.     

Little Brown Bats (Myotis lucifugus) Recognize Individual Identity of Conspecifics Using Sonar Calls

Bats use sonar calls to locate prey and orient in their environment but they may also be used by conspecifics to obtain information about a caller. Statistical analysis of sonar calls provides evidence that variation carries social information about a caller, including individual identity. We hypothesized that little brown bats (Myotis lucifugus) would be able to recognize individuals given the potential fitness benefits of doing so. We performed playback trials using a habituation‐discrimination design to determine whether little brown bats are able to recognize the individual identity of a caller based on variation in their sonar calls. Each subject bat was played the calls of bat A until they habituated (defined as a 50% decrease from the beginning call rate), then the calls of bat B or a new call sequence of bat A (a control, referred to as bat A’) were played. Each subject received a unique pair of playback recordings (bat A and B) from adult female bats from the same colony (but a different colony from the subject) and the order of trials was randomized. The response measures were habituation time (s) and call rate (calls/s). Within a trial, subjects habituated to calls of bat A and transferred this habituation to the bat A’ sequence. In addition, they increased their call rates when played calls of bat B. Comparing between trials, subjects increased their call rate to the calls of bat B to a greater relative extent than to the calls of bat A’. These results provide the first evidence that bats recognize individual identity of conspecifics (as opposed to discrimination of groups), which has implications for the social interactions of bats.     

On the responses of laughing gull chicks (Larus atricilla) to the calls of adults I. Recognition of the voices of the parents

Twelve matched pairs of parent-raised and hand-raised laughing gull chicks from the same clutches were tested, at 6 to 8 days post-hatching, with successive presentations of playback of a recording of calls of their parents and recording of calls of parents from a neighbouring nest, in a situation in which they could approach or withdraw from the sound.The parent-raised chicks oriented towards the sound, moved towards the sound and called when the calls of their own parents were played; they tended to withdraw from the sound and sit silently when the calls of the neighbouring parents were played.The hand-raised chicks fled from the sound and crouched in silence in response to both calls of their parents and the calls of neighbours. They were more vocal and active in periods without sound than were the parent-raised chicks.It is concluded that laughing gull chicks in nature learn to recognize the individual characteristics of the calls of their parents, and to react positively to the calls, through interaction with their parents prior to the age of 6 days post-hatching.     

Age Differences in the Response of Willow Tits (Parus montanus) to Conspecific Alarm Calls

Predation is an important mortality factor in wintering birds. To counter this, birds produce alarm calls in the presence of predators which serve to warn conspecifics. In social hierarchical bird flocks, adults survive the winter better than juveniles and therefore survival strategies probably vary with social status. This study examined the differential responses to alarm calls by free-living willow tits, Parus montanus, in dominance-structured winter flocks in Finland. To explore the age-dependent differences in response to conspecific alarm calls, a series with three alarm calls was played to focal adults and juveniles while they sat in the middle section of a spruce branch. Immediately after the playback, juvenile willow tits moved more often, flew longer distances and changed branches more often than did adults. Previous mammal studies have shown that juveniles are more likely to flee than adults after hearing conspecific alarm calls. The current study demonstrates that similar age-dependent responses to conspecific alarm calls occur in birds also. These findings reflect an increased vulnerability to predators or lack of experience of young birds.     

Female House Wrens may Use a Low‐Amplitude Call as an Aggressive Signal

The majority of work on aggressive signaling has focused on male–male contests. Although females in many species compete over important resources, female signals are understudied. In house wrens (Troglodytes aedon), females compete with other females to protect nesting cavities and eggs. We suggest that a high‐pitched, low‐amplitude call, hereafter abbreviated as ‘HI’, may be an aggressive signal used by female house wrens. Using playback of simulated female intruders, we found that females used HI calls significantly more often during playback of female house wrens compared to playback of heterospecifics. Additionally, HI calls were given significantly more often in the minute preceding physical attack. In comparison, song rates did not predict future attacks. Finally, we present pilot data that suggest listening females may respond more aggressively to female playback containing HI calls compared to playback containing only songs. This suggests the reliability of HI calls could be maintained by a receiver retaliation rule. HI calls bear a striking resemblance to the low‐amplitude songs of many male songbirds, in terms of both acoustic structure and social context. This is one of the few reports of a putative low‐amplitude aggressive signal in a female songbird. However, the nature of female competition may make low‐amplitude signaling an underappreciated signaling form in female animals.     

Characteristics of the Signals of Male <Emphasis Type="Italic">Anoeconeossa bundoorensis</Emphasis> Taylor and Burckhardt (Hemiptera: Aphalaridae) Associated with Female Responsiveness

Sexual selection requires intra-specific variation in the characteristics mediating mate choice. In species reliant on substrate-borne vibrational signalling (SBVS), differences in the attractiveness of individual signallers’ calls can influence relative mating success since they can indicate the quality of the sender. We used laser vibrometry and playback experiments to study duet signalling in a psyllid (Anoeconeossa bundoorensis Taylor and Burckhardt), in particular to identify characteristics linked to female responsiveness. Signals were sex-specific with the syllables (or calls) of the smaller males attaining higher frequencies than the syllables of the larger females. Male syllables build over time with more energy in the second half of the call while those of females have a more uniform energy distribution. Male syllables vary in the timing of the halfway point of energy production. We used playback to examine species recognition and female responsiveness to the first and second halves of male calls. We demonstrated that females responded only to calls of male A. bundoorensis and that they were more likely to respond to the first half of male calls. Based on the results of our second playback experiment, we found that a gradation in energy production is more important for eliciting a female reply than a uniform distribution of energy. Our findings show that the rate of energy production facilitates differentiation between syllables which could be used to indicate the quality of individual males as mates.     

Males of <Emphasis Type="Italic">Hypsiboas goianus</Emphasis> (Anura; Hylidae) do not assess neighbor fighting ability through acoustic interactions

In frogs, acoustic signals are the most important communication mechanism, since they may be used in several social contexts. In many anurans, the dominant frequency of calls is negatively related to body size, and in such species, this spectral parameter may be considered a good predictor of fighting ability. We experimentally investigated the vocal behavior of 30 male Hypsiboas goianus in central Brazil to answer the following questions: (1) Do males change the acoustic parameters of their calls in response to conspecific intruders? and (2) Does the acoustic behavior of H. goianus depend on the simulated body size of their opponent? We used playback of synthesized calls with high (3573 Hz) and low (3123 Hz) dominant frequency to simulate small and large males, respectively. Males reduced the rate of advertisement calling in response to playback but did not change vocal behavior in response to low-frequency and high-frequency playback. So, while males adjust their calling activity in response to simulated conspecifics, there was no evidence that they assess the fighting ability of their opponents through acoustic interactions.     

Female preference for fast-rate, high-pitched calls in Hermann's tortoises Testudo hermanni

The courtship and mounting behavior of promiscuous tortoisesis based on a multiple signaling system that includes visual,olfactory, tactile, and acoustic signals. Vocalizations relatedto mounting seem to be particularly intriguing because tortoisesvocalize mainly at this time. Vocalizations and courtship behaviormight be costly for males, and if these costs increase differentiallyfor different males, then the potential exists for vocalizationsand displays to reveal male individual quality on which femalescould base their mate choice. In the present study, by usingindividuals breeding in seminatural enclosures, we experimentallyinvestigated whether the mounting call of male Hermann's tortoises(Testudo hermanni) represents a relevant stimulus to both malesand females. We also looked for potential preference towarddifferent acoustic stimuli in order to determine which callfeatures, if any, played a role in attracting potential partnersor deterring rivals. Females were sensitive to call playback,whereas males apparently paid little attention to playback.In addition, females showed a significant preference towardhigh-pitched calls emitted at high rate. Also, females exhibiteda strong tendency toward short-duration calls. Because callrate is related to hematocrit in Hermann's tortoise males, byresponding to fast-rate calls females are also choosing high-qualitypartners as sires of their offspring. This is the first studyin which an attractive function of tortoise vocalizations hasbeen documented.     

Assessing aural and visual cueing as tools for seabird management

Social attraction, that is, mimicking of active and productive colonies via audio playback of calls of breeding conspecifics and the use of decoys, is commonly used to attract birds to newly established or restored breeding sites. However, little is known about the relative importance of aural versus visual cues for identify nesting areas. Such information is important for design and evaluation of management protocols. We studied the effectiveness of decoys (visual cues) and playbacks (audio cues) as methods for restoring a colony of common terns (Sterna hirundo) at Muskeget Island, Massachusetts, USA. We used a 2-year, crossover experiment with 3 treatment areas: audio and visual, audio only, and visual only. We reversed treatment areas in the second year to control for previous nesting area or substrate preference. In both years, nests were built 9–101 m downwind of loudspeakers. There was no overlap in areas used for nesting between years and no nests were built within decoy plots in either year. Behavioral observations showed that birds responded to decoys only when within range of sound treatments. Conspecific vocalizations appear to be important proximate cues for seabird colony site selection and should be given priority in management protocols using social attraction. © 2011 The Wildlife Society.     

Absentee Mothering – Not So Absent? Responses of European Rabbit (Oryctolagus cuniculus) Mothers to Pup Distress Calls

Longer‐range acoustic parent‐offspring communication is widespread, but might be absent in species in which young are hidden in burrows during the mother's absence. The European rabbit (Oryctolagus cuniculus) is such a species, with unusually limited maternal care largely restricted to a short daily nursing bout. Based, however, on evidence of frequent infanticide in this species, we hypothesize that rabbits possess a mechanism promoting a maternal response to pup distress calls. We conducted playback experiments with distress calls of pre‐weaning pups played next to the breeding burrows of mothers in a field enclosure (i.e. next to the burrows where mothers give birth and raise their young). Calls were played shortly after pups were born (T1) when infanticide risk is maximal, and shortly before the pups start dispersing from the breeding burrow (T2). A high proportion of mothers (60.6%) responded to pup calls by rapidly returning to their breeding burrow and 40% of them investigated the area around the entrance. Return responses to the playback of pup calls did not differ between mothers during T1 and T2. Thus, our results confirm that rabbit mothers respond rapidly to pup distress calls and that this responsiveness may adaptively serve to repel potentially infanticidal females.     

Female preference functions based on call duration in the gray tree frog (Hyla versicolor)

Preference functions, which quantify preference strength relativeto variation in male traits or signals, are central to understandingmechanisms and consequences of female choice. Female tree frogs(Hyla versicolor) choose mates on the basis of advertisementcalls and prefer long calls to short calls. Here we show, intwo experimental designs, that preference strength increasedsignificantly as the difference in call duration was increasedonly if the absolute durations of alternative stimuli were below average. Hence preference strength was a non-linear functionof duration, and females did not base preferences solely onthe percentage difference in duration. In experiments simulatingcostly choice (unequal playback levels), non-linear effectswere more pronounced than in the conventional design (equal playback levels). Repeated estimates of preference strengthusing the unequal-playback design revealed significant among-femalevariation. These patterns of preference suggest that selectionby female choice for males producing calls of average durationover males producing very short calls is stronger than selectionfor males producing very long calls over males producing callsof average duration. Female preferences, especially in tests simulating a potentially costly choice, could reflect differencesin the net benefits to females of mating with males producingcalls of different duration.     

Functionally similar acoustic signals in the corncrake (Crex crex) transmit information about different states of the sender during aggressive interactions

We combined playback experiments with hormonal manipulations to study the information content of acoustic signals during aggressive interactions between male corncrakes. During territorial conflicts, fights are uncommon, but the intensity of signaling usually increases. Such signals can be temporally and contextually associated with many aggressive behaviors and most likely function as threats or as indicators of the sender's quality or motivation. However, such correlational data are unsatisfactory for the proper interpretation of the function and information content of signals. Experimental tests are required to determine whether signals and aggressive behaviors are controlled by common or independent mechanisms. In our experiment, we assigned subjects to four groups: testosterone-implanted birds, flutamide-implanted birds, birds with empty implants, and non-captured control birds. Males produced two types of calls (quiet soft calls and loud broadcast calls), both of which are known to be reliable predictors of aggressive escalation. When testosterone action was blocked with flutamide, males significantly limited the amount of time spent close to the playback speaker and stopped responding to playback with soft calls. Broadcast calling was unaffected by the blockage of testosterone. Conversely, increased levels of testosterone neither affected calling nor the time spent near the speaker, indicating a permissive, rather than a graded effect of androgens. We concluded that, despite the seemingly similar function, both signals may transmit information about different states of the sender; soft calls seem to imply a threat of force, while broadcast calls appear to be more similar to an announcement, which is only indirectly associated with a male's aggressive behavior.     

Synchronized interdigitated calling in the Kuvangu running frog, Kassina kuvangensis

Male Kuvangu frogs show repetitive calling of pulsed advertisement calls in which up to seven calls are repeated in short succession. Recordings of pairwise interactions between males showed that calls were highly synchronized, with individual calls interdigitating with each other. Males frequently switched between the leader and follower role with neither male dominating the interaction. Interactive playback experiments using synthetic calls revealed that males slightly but significantly increased the number of calls per call group with increases in stimulus call number. Males also significantly increased call rate with the number of calls in the playback stimulus. Furthermore, when presented with shortened intercall intervals, males increased their own intercall intervals, thus ‘skipping’ a call and avoiding overlap with the playback. The low degree of call matching suggests that repetitive calling, apart from maintaining a male's attractiveness to females relative to rival males, may mediate male-male competition. In addition, synchronized interdigitated calling may serve to reduce predation, while maintaining species-specific temporal features of advertisement calls important to females. Kuvangu running frogs may have reduced the costs associated with synchrony and alternation by using a signal timing scheme that allows them to do both simultaneously.Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.       

Variable effect of playback of chickadee mobbing calls on detection probability of boreal forest birds

Modification of the point count survey method to include playback of songbird mobbing calls in an attempt to increase detection probabilities has met with mixed success. We compared detection probabilities for boreal forest songbirds using traditional point count methods and counts using broadcasts of the mobbing calls of Black‐capped Chickadees (Poecile atricapillus) in an attempt to increase detection probability. We conducted 594 point counts during the 2010 breeding season in Newfoundland, Canada. Each point count consisted of an 8‐min silent observation period followed by an 8‐min broadcast of Black‐capped Chickadee mobbing calls. Occupancy model results showed that response to playback broadcast varied across species, with detection probabilities higher for seven of 17 species during the silent portions of point counts and three species more likely to be detected during playback intervals. For all species, the number of visual detections increased during periods of playback and, averaged across species, individuals were >6 times more likely to be seen during the playback period than during the silent period. Differences in detection probability among observers were apparent during both silent and playback periods. We suggest that using playback of chickadee mobbing calls during point count surveys of common boreal forest songbird species may be most beneficial when visual detection is important. However, playback may also be useful for species‐specific surveys during periods when birds are less likely to be vocal or for studies of less common species with chronically low detection probabilities. A combined silent and playback approach could also be useful, although observer and species differences should be accounted for if comparing data across species or studies.     

Wild fledgling tits do not mob in response to conspecific or heterospecific mobbing calls

Mobbing, where birds harass a predator through a combination of vocalizations and stereotyped behaviours, is an effective anti-predator behaviour for many species. Mobbing may be particularly important for juveniles, as these individuals are often more vulnerable than adults. Although the component behaviours of mobbing are often considered to be un-learned, there are few confirmatory data, and the developmental trajectory of mobbing is unknown. In this study, we tested whether conspecific or heterospecific mobbing calls initiated mobbing behaviour in juvenile Blue Tits Cyanistes caeruleus. We located wild adult and recently fledged juvenile Blue Tits and presented them with playback recordings of adult conspecific (Blue Tit) and heterospecific (Great Tit Parus major) mobbing alarm calls. Although adult birds readily mob in response to these types of playbacks, juveniles did not exhibit characteristic mobbing behaviour. Some juveniles did, however, exhibit individual components of mobbing behaviour found in mobbing, despite not producing adult-like mobbing behaviour in response to either conspecific or heterospecific playback. These results suggest that, although birds might be capable of mobbing as juveniles, the associations between the non-vocal stereotyped mobbing behaviours and mobbing calls may be learned.     

Dual Function of Chip Calls Depending on Changing Call Rate Related to Risk Level in Territorial Pairs of White‐Eared Ground‐Sparrows

Different mechanisms have been proposed for encoding information into vocalizations: variation of frequency or temporal characteristics, variation in the rate of vocalization production, and use of different vocalization types. We analyze the effect of rate variation on the dual function of chip calls (contact and alarm) produced by White‐eared Ground‐sparrows (Melozone leucotis). We conducted an acoustic playback experiment where we played back 1 min of four chip call rates (12, 36, 60, 84 calls/min). We measured the response of territorial pairs using behavioral responses, and fine structural features of calls produced in response to those playbacks. White‐eared Ground‐sparrows showed more intense behavioral responses to higher than lower call rate playbacks. Both individuals of the pair approached the source of the playback stimulus faster, produced the first vocalization faster, produced more vocalizations, and spent more time close to the stimulus in higher call rate than in lower call rate playbacks. Frequency and duration characteristics of calls (chip and tseet) were similar in response to all call rate playbacks. Our playback experiment elicited different intensity of behavioral responses, suggesting that risk‐based information is encoded in call rate. Our results suggest that variation in the rate of chip call production serves a dual function in this species; calls are used at lower rates for pair contact and at higher rates for alarm/mobbing signals.     

Sex-specific territorial responses in Tawny Owls Strix aluco

A territorial intruder often poses more of a threat to the territory holder of the same sex as itself. As territorial aggression is associated with costs, aggression shown by birds towards territorial intruders of the opposite sex deserves investigation. This behaviour could arise due to the reproductive value of a mate or through mutualism between members of a pair. We investigated these hypotheses by presenting mated pairs of Tawny Owls Strix aluco with playback of male calls, female calls and a male and female duetting, and recording the number and intensity of responses by the male and female territory holders. Females responded significantly more often to female than to male calls. Males responded equally often to male and female playback. Males which had previously bred successfully with their mate were significantly more likely to respond to female playback in the spring, which suggested males were responding to female playback due to the reproductive value of their mate. There was no evidence of mutualism between members of a pair.     

AUDIO COMPUTERS—THEORY OF OPERATION AND GUIDELINES FOR SELECTION OF SYSTEMS AND COMPONENTS

ABSTRACT

Computers are available that can store, synthesize and replay sounds using digital technology. I describe the major components of audio computers, the principles of digital sound acquisition and playback, and information and caveats for scientists interested in acquiring an audio computer system for their own use. A tutorial on analog filter application is also included as well as a diagnostic procedure and a buyer's check list.     

Singing in the Face of Danger: the Anomalous Type II Vocalization of the Splendid Fairy-Wren

Males of certain species of fairy-wrens (Aves: Maluridae) emit a unique vocalization, the Type II vocalization, in response to the calls of potential predators. We conducted field observations and playback experiments to identify the contexts in which the Type II vocalization is emitted by splendid fairy-wren ( Malurus splendens ) males, and to examine social and genetic factors that influence its occurrence. In field observations and controlled playback experiments, Type II vocalizations were elicited most consistently by calls of the predatory gray butcherbird ( Cracticus torquatus ). Some vocalizations from other avian species also elicited Type II vocalizations, and the majority of these were vocalizations from avian predators. Splendid fairy-wrens are cooperative breeders, and males that responded with Type II vocalizations to playbacks of butcherbird calls tended to be primary rather than secondary males, had larger cloacal protuberances, and were older than those that did not respond. In addition, secondary males that were sons of resident females were more likely than non-sons to respond with a Type II vocalization. In another playback experiment, females responded similarly to the Type I song and Type II vocalizations of their mates. Although the Type II vocalization is emitted primarily in response to predator calls, it is inconsistent with an alarm call explanation. Patterns of reproductive success among Type II calling males suggest that it does not function as an honest signal of male quality. At present, the function of the vocalization remains anomalous, but indirect fitness benefits may play a role in its explanation.     

Does begging call convergence increase feeding rates to nestling tree swallows Tachycineta bicolor?

Some studies suggest that offspring might coordinate their begging displays to send a more effective brood signal, which in turn, could increase parental feeding rates. In tree swallows Tachycineta bicolor , when nestlings call together, their calls are more similar in structure than when they call alone. Here, we tested the hypothesis that call convergence enhances the overall brood signal, thus increasing parental provisioning rates. We played back similar and dissimilar calls (as measured by cross-correlation) to parents during a one-h playback period, and filmed the response of parents and nestlings. Contrary to our hypothesis, parental feeding rates did not differ in relation to call similarity. Based on these results, call similarity does not appear to function as a coordinated brood signal in tree swallows.