Classification threshold and training data affect the quality and utility of focal species data processed with automated audio-recognition software

ABSTRACT

Automated recognition is increasingly used to extract information about species vocalizations from audio recordings. During processing, recognizers calculate the probability of correct classification (“score”) for each acoustic signal assessed. Our goal was to investigate the implications of recognizer score for ecological research and monitoring. We trained four recognizers with clips of Common Nighthawk (Chordeiles minor) calls recorded at different distances: near, midrange, far, and mixed distances. We found distance explained 49% and 41% of the variation in score for the near and mixed-distance recognizers, but only 3% and 6% of the variation for the midrange and far recognizers. We calculated detection functions for each of the recognizers at various score thresholds and found that the detection function for the near and mixed-distance recognizers satisfied the assumptions of density estimation for most score thresholds, while the detection function for the midrange and far recognizers did not. The detection functions also showed that score threshold choice is a decision about sampling area, not just about the balance between recall and precision. Overall, we showed that training recognizers with ‘high-quality’ clips that were recorded at close range will improve the utility of the data without affecting how many true positives the recognizer detects.     

Towards the automated detection and occupancy estimation of primates using passive acoustic monitoring

Recent advancements in technology have made possible the use of novel, cost-efficient biomonitoring techniques which facilitate monitoring animal populations at larger spatial and temporal scales. Here, we investigated using passive acoustic monitoring (PAM) for wild primate populations living in the forest of Taï National Park, Côte d’Ivoire. We assessed the potential of using a customized algorithm for the automated detection of multiple primate species to obtain reliable estimates of species occurrence from acoustic data. First, we applied the algorithm on continuous rainforest recordings collected using autonomous recording units (ARUs) to detect and classify three sound signals: chimpanzee buttress drumming, and the loud calls of the diana and king colobus monkey. Using an occupancy modelling approach we then investigated to what extent the automated, probabilistic output needs to be listened to, and thus manually cleaned, by a human expert, to approach occupancy probabilities derived from ARU data fully verified by a human. To do this we explored the robustness of occupancy probability estimates by simulating ARU datasets with various degrees of cleaning for false positives and false negative detections. We further validated the approach by comparing it to data collected by human observers on point transects located within the same study area. Our study demonstrates that occurrence estimates from ARU data, combined with automated processing methods such as our algorithm, can provide results comparable to data collected by humans and require less effort. We show that occupancy probabilities are quite robust to cleaning effort, particularly when occurrence is high, and suggest that for some species even naïve occupancy, as derived from ARU data without any cleaning, could provide a quick and reliable indicator to guide monitoring efforts. We found detection probabilities to be most influenced by time of day for chimpanzee drums while temperature and, likely, poaching pressure, affected detection of diana monkey loud calls. None of the covariates investigated appeared to have strongly affected king colobus loud call detection. Finally, we conclude that the semi-automated approach presented here could be used as an early-warning system for poaching activity and suggest additional techniques for improving its performance.     

Optimizing detection of the bobwhite reproduction call using passive acoustic monitoring

Northern bobwhite (Colinus virginianus) populations have declined across much of their range. In response to these declines, wildlife biologists and managers have increased survey efforts and tried to optimize detection and capitalize on technological advances to improve population estimates and cost-effectiveness. Our objective was to determine how environmental conditions influence detection of the reproduction call, or whistle, of masked bobwhite (C. v. ridgwayi), an endangered subspecies of northern bobwhite, using autonomous recording units (ARUs). We estimated the call intensity of the masked bobwhite reproduction call as 112 ± 0.5 decibels (mean ± SE) at 10 cm. We then broadcasted 16,284 calls during 17 trials to compare manual and automated call detection in recordings collected with ARUs. We used these data to model detectability of a bobwhite reproduction call, for when the bird is present and available, as a function of distance and weather conditions using generalized linear mixed models with trial as a random effect. Regardless of detection type, one model structure was competitive and suggested detection probability was a function of distance, wind speed, and wind direction. Detectability decreased with increased distance and wind speed and was influenced by wind direction. We demonstrate the use of our results to predict the probability of detecting a reproduction call during ARU-based monitoring efforts. By understanding the effects of environmental factors on the detection of a bobwhite reproductive call, bobwhite surveys can be improved.     

Acoustic detection and acoustic habitat characterisation of the critically endangered white-bellied heron (Ardea insignis) in Bhutan

1. Growing developmental activities, such as hydropower construction, farm roads, and other human activities, are affecting the critically endangered white-bellied heron (WBH). Out of a known global population of 60, 28 individuals inhabit the river basin area and freshwater lakes and ponds of Bhutan. Several constraints impede continuous monitoring of endangered species, such as the isolated and cryptic nature of the species and the remoteness of its habitat; to date, there are no long-term reference data or techniques implemented for continuous monitoring of this species.
2. In this study, we designed acoustic detection and habitat characterisation methods using long-duration recordings from three habitat areas in Bhutan. Acoustic indices were extracted and used to implement a species-specific call detector and to generate habitat soundscape representations. Using WBH calls annotated in month-long recordings from a known site, a novel indices-based detector was implemented and tested. A total of 960 hr of continuous audio recordings from three habitats in Bhutan were analysed.
3. We found that a species call detector implemented using a combination of acoustic indices (that includes measures of spectral and temporal entropy and different angles of spectral ridges) has a correct detection rate of 81%. Additionally, visual inspection of the species’ acoustic habitat using long-duration false-colour spectrograms enabled qualitative assessment of acoustic habitat structure and other dominant acoustic events.
4. This study proposes a combined approach of species acoustic detection and habitat soundscape analysis for holistic acoustic monitoring of endangered species. As a direct outcome of this work, we documented acoustic reference data on the critically endangered WBH from multiple habitat areas and have analysed its temporal vocalisation patterns across sites.

     

Humans versus autonomous recording units: a comparison of point‐count results

ABSTRACT There is growing interest in the potential use of autonomous recording units (ARUs) to obtain point‐count bird survey data in the absence of human observers. To determine possible reasons for differences in the point‐count performances of humans and ARUs and to better understand the possible limitations of ARUs, we compared the results of point‐counts conducted by human observers with those conducted using ARUs. Human observers and ARUs recorded birds at 56 point‐count stations in three different habitats (green mixed‐conifer forest, burned mixed‐conifer forest, and mixed riparian cottonwood bottomland). Combined, human observers and ARUs generated a total of 858 detections of 86 different species. We found that 9.7% of detections were recorded by ARUs only, 40.9% by human observers only, and 49.4% by both ARUs and humans. The mean number of species detections per point was significantly greater for human observers (13.8) than for ARUs (9.1), and did not differ significantly among habitat types. Birds not detected using ARU recordings included those with songs or calls too distant to be recorded (52.7%), those detected by human observers using only visual cues (14.8%), and those too difficult to identify from recordings (10.3%) or simply overlooked (8.8%). About two‐thirds (68.7%) of birds detected using ARUs, but missed by human observers in the field, were simply overlooked in the field; most of the rest were the result of misidentification in the lab. The failure of ARUs to record a large proportion of the detections recorded by human observers, combined with problems with detecting or identifying sounds in the lab and the extra time and cost associated with use of ARUs, suggest that they would not provide a cost‐effective means of gathering data for traditional point‐count surveys.     

Using bioacoustics to enhance the efficiency of spotted owl surveys and facilitate forest restoration

The California spotted owl (Strix occidentalis occidentalis) is an older-forest associated species that resides at the center of forest management planning in the Sierra Nevada and Southern California, USA, which are experiencing increasingly large and severe wildfires and drought-related tree mortality. We leveraged advances in passive acoustic survey technologies to develop an acoustically assisted survey design that could increase the efficiency and effectiveness of project-level surveys for spotted owls, allowing surveys to be completed in a single year instead of in multiple years. We deployed an array of autonomous recording units (ARUs) across a landscape and identified spotted owl vocalizations in the resulting audio using BirdNET. We then evaluated spatio-temporal patterns in spotted owl vocalizations near occupied territories and the ability of a crew naïve to the location of occupied territories to locate spotted owls based on patterns of acoustic detections. After only 3 weeks of acoustic surveys, ≥1 ARU within 750 m of all 17 occupied territories obtained spotted owl detections across ≥2 nights. When active surveys using broadcast calling were conducted near ARUs with spotted owl detections by surveyors naïve to territory occupancy status and locations, surveyors located owls in 93% to 100% of occupied territories with ≤3 surveys. To further improve the efficiency of spotted owl surveys, we developed a statistical model to identify and prioritize areas across the Sierra Nevada for different survey methods (active only, acoustically assisted, no surveys) based on the expected probability of occupancy predicted from remotely sensed measurements of tree height and historical occupancy. Depending on managers' tolerance for false negatives, this model could help identify large areas that might not benefit from surveys based on low expected occupancy probabilities and areas where acoustically assisted surveys might enhance survey effectiveness and efficiency. Collectively, these findings can help managers streamline the survey process and thus increase the pace of forest restoration while minimizing potential near-term adverse effects on California spotted owls.     

Response of Hawaiian Moorhens to broadcast of conspecific calls and a comparison with other survey methods

ABSTRACT Accurate population surveys are critical for effective management of species of conservation concern. Traditional visual and aural surveys are ineffective for behaviorally cryptic species such as rails, but incorporating call‐broadcast into surveys can increase their detection rate. From 2004 to 2006, we surveyed wetlands (N= 67) on Kauai and Oahu for endangered Hawaiian Moorhens (Gallinula chloropus sandvicensis) with the goal of comparing the effectiveness of visual and aural observations to that of call‐broadcast surveys. We evaluated six different Hawaiian Moorhen calls, including the “squeal” call of young moorhens. We also compared the results of surveys conducted using the broadcast of Hawaiian Moorhen calls to those of extended time (75 min) surveys and surveys where the calls of the North American subspecies of moorhen (G. c. cachinnans) were broadcast. We found that broadcast of Hawaiian Moorhen calls increased detection rates by 56% on Kauai and 30% on Oahu. Territorial and chick‐distress calls elicited the greatest response. We also found a nonlinear positive relationship between the estimated population of Hawaiian Moorhens at a wetland and the improvement in detection due to call‐broadcast, suggesting social facilitation of responses. Survey periods of 60 min produced results similar to those obtained using call‐broadcast. However, long survey periods require more time than call‐broadcast surveys and increase the likelihood of double counting individuals. Broadcast of the calls of the North American subspecies of moorhen failed to increase detection rates above those obtained using visual and aural surveys. Our results suggest that the population of Hawaiian Moorhens is larger than previously estimated, but is likely well below the 2000 individuals recommended for removal from the Endangered Species list. We recommend the use of call‐broadcast during surveys of Hawaiian Moorhens to improve estimates of population sizes and trends.     

Traffic noise differentially impacts call types in a Japanese treefrog (Buergeria japonica)

Acoustic noise from automobile traffic impedes communication between signaling animals. To overcome the acoustic interference imposed by anthropogenic noise, species across taxa adjust their signaling behavior to increase signal saliency. As most of the spectral energy of anthropogenic noise is concentrated at low acoustic frequencies, species with lower frequency signals are expected to be more affected. Thus, species with low-frequency signals are under stronger pressure to adjust their signaling behaviors to avoid auditory masking than species with higher frequency signals. Similarly, for a species with multiple types of signals that differ in spectral characteristics, different signal types are expected to be differentially masked. We investigate how the different call types of a Japanese stream breeding treefrog (Buergeria japonica) are affected by automobile traffic noise. Male B. japonica produce two call types that differ in their spectral elements, a Type I call with lower dominant frequency and a Type II call with higher dominant frequency. In response to acoustic playbacks of traffic noise, B. japonica reduced the duration of their Type I calls, but not Type II calls. In addition, B. japonica increased the call effort of their Type I calls and decreased the call effort of their Type II calls. This result contrasts with prior studies in other taxa, which suggest that signalers may switch to higher frequency signal types in response to traffic noise. Furthermore, the increase in Type I call effort was only a short-term response to noise, while reduced Type II call effort persisted after the playbacks had ended. Overall, such differential effects on signal types suggest that some social functions will be disrupted more than others. By considering the effects of anthropogenic noise across multiple signal types, these results provide a more in-depth understanding of the behavioral impacts of anthropogenic noise within a species.     

Improving geographically extensive acoustic survey designs for modeling species occurrence with imperfect detection and misidentification

Acoustic recording units (ARUs) enable geographically extensive surveys of sensitive and elusive species. However, a hidden cost of using ARU data for modeling species occupancy is that prohibitive amounts of human verification may be required to correct species identifications made from automated software. Bat acoustic studies exemplify this challenge because large volumes of echolocation calls could be recorded and automatically classified to species. The standard occupancy model requires aggregating verified recordings to construct confirmed detection/non‐detection datasets. The multistep data processing workflow is not necessarily transparent nor consistent among studies. We share a workflow diagramming strategy that could provide coherency among practitioners. A false‐positive occupancy model is explored that accounts for misclassification errors and enables potential reduction in the number of confirmed detections. Simulations informed by real data were used to evaluate how much confirmation effort could be reduced without sacrificing site occupancy and detection error estimator bias and precision. We found even under a 50% reduction in total confirmation effort, estimator properties were reasonable for our assumed survey design, species‐specific parameter values, and desired precision. For transferability, a fully documented r package, OCacoustic, for implementing a false‐positive occupancy model is provided. Practitioners can apply OCacoustic to optimize their own study design (required sample sizes, number of visits, and confirmation scenarios) for properly implementing a false‐positive occupancy model with bat or other wildlife acoustic data. Additionally, our work highlights the importance of clearly defining research objectives and data processing strategies at the outset to align the study design with desired statistical inferences.     

Habitat complexity and the structure of vocalizations: a test of the acoustic adaptation hypothesis in three rail species (Rallidae)

The acoustic adaptation hypothesis is based on the assumption that senders are directionally selected to maximize transmission and minimize degradation; however, the two aims are not necessarily convergent. In complex habitats, where more effects that might potentially cause attenuation and degradation co‐occur and longer transmission must incur a higher cost, signals should attenuate faster and have shorter transmission ranges. At the same time, such signals should be more resistant to degradation in order to preserve their communicatory function. Based on a sound transmission experiment, we tested the evidence for these predictions using territorial calls of three sympatric species of rails, inhabiting habitats with increasing complexity: Corncrake Crex crex, Spotted Crake Porzana porzana and Water Rail Rallus aquaticus. In the experiment, the calls were broadcasted with similar amplitudes through a heterogeneous habitat inhabited by all three species and rerecorded at different distances up to 320 m. Despite standardized amplitudes and habitats, calls of the species living in simpler habitats had longer transmission ranges but were more susceptible to degradation than calls of the species living in more complex habitats. Our results suggest that narrow frequency bandwidth is an adaptation of species inhabiting complex habitats that helps their calls to degrade less at the cost of stronger attenuation and shorter transmission range. By contrast, wide frequency bandwidth extends the range but increases degradation and thus it is used only by species inhabiting structurally simpler habitats. This study shows that, in more complex habitats, the clarity of the message is preserved at the cost of range.     

Inter- and Intraspecific Variation in the Acoustic Habitats of Three Marmot Species

Closely related species often have remarkably different vocalizations. Some of the variation in acoustic structure may result from species adapting their calls to maximize transmission through their acoustic environments. We document the relative magnitude of inter- and intraspecific variation in acoustic transmission properties of the habitats of three closely related marmot species to study the relative importance that the acoustic environment may have played in selecting for species-specific marmot alarm calls. We used spectrogram correlation to quantify the degree to which pure tones and alarm calls changed as they were broadcast through marmot home ranges to describe the acoustic habitats of golden (M. candata aurea), yellow-bellied (M. flaviventris), and alpine (M. marmota L.) marmots. Species lived in quantifiably different acoustic habitats. One analysis partitioned variation between species and between marmot social groups (nested within species). We found significant interspecific variation in the acoustic transmission fidelity of the three species' habitats and insignificant intraspecific variation between social groups. Further analysis of a larger sample of alarm calls broadcast through golden marmot social groups found significant intraspecific variation. Interspecific variation greater than intraspecific variation suggests that variable acoustic habitats may be responsible for at least some of the interspecific variation in alarm call structure. This is the first study to use spectrogram correlation to describe habitat acoustics. We discuss aspects of the method that may be useful for others seeking to quantify habitat acoustics.     

Does Gray Squirrel (Sciurus carolinensis) Response to Heterospecific Alarm Calls Depend on Familiarity or Acoustic Similarity?

In habitats in which multiple species are prey to the same predators, individuals can greatly benefit from recognizing information regarding predators that is provided by other species. Past studies have demonstrated that various mammals respond to familiar heterospecific alarm calls, but whether acoustic similarity to a familiar call can prompt a mammal's recognition of an unfamiliar call has yet to be shown. We presented alarm calls to free‐ranging eastern gray squirrels (Sciurus carolinensis) and recorded behavioral changes in vigilance and antipredatory response. Playbacks included alarm calls of a sympatric bird (American robin, Turdus migratorius), an allopatric bird with a call structure similar to that of the robin (common blackbird, Turdus merula), and an allopatric bird with a distinct call structure (New Holland honeyeater, Phylidonyris novaehollandiae). Squirrels responded significantly more frequently to squirrel alarm calls (positive control) than to robin song (negative control) or honeyeater calls. Squirrel response to robin and blackbird alarm calls was statistically similar to their response to squirrel alarm calls, indicating that squirrels responded to those alarm calls as if they provided information about the presence of predators. However, squirrel response to robin song was not statistically different from response to any of the other avian calls, including the robin and blackbird alarms, suggesting that squirrels neither respond to blackbird alarms as if they clearly signify danger, nor as if they clearly do not signify danger, perhaps reflecting some ambiguity in interpretation of the calls. These results suggest that squirrel responses to alarm calls are generally based on call familiarity, but that acoustic similarity of an unfamiliar allopatric call to a familiar call also can elicit antipredator behavior. The lack of response to honeyeater alarm calls also supports the hypothesis that call recognition by gray squirrels is dependent on familiarity, rather than simply detection of an acoustic feature common to alarm calls across a variety of avian species.     

Sex identification in King Penguins Aptenodytes patagonicus through morphological and acoustic cues

In the context of sexual selection, animals have developed a variety of cues conveying information about the sex of an individual to conspecifics. In many colonial seabird species, where females and males are monomorphic and do not show obvious differences in external morphology, acoustic cues are an important signal for individual and sex recognition. Here, we study the vocal and morphological sex dimorphism in the King Penguin Aptenodytes patagonicus, a colonial, monomorphic seabird for which our knowledge about the role of vocalizations and morphology in mate choice is very limited. Data were collected at Possession Island, Crozet Archipelago, in a breeding colony consisting of about 16 000 breeding pairs. Using measurements of six morphological features and analysing acoustic parameters of call recordings of adult individuals, we show that King Penguins can be sexed based on a single morphological measurement of the beak with an accuracy of 79%. We found a sex‐specific syntax in adult King Penguin calls that provided a 100% accurate method to distinguish between the sexes in our study population. To confirm the method at the species level, we analysed calls recorded from King Penguin adults in Kerguelen Island, 1300 km away from our study population and found the same accuracy of the sex‐specific syntax. This sex‐specific syllable arrangement is rare in non‐passerines and is a first step in understanding the mate choice process in this species. Furthermore, it offers a cost‐effective, non‐invasive technique for researchers to sex King Penguins in the field.     

The use of Vocalizations of the Sambirano Mouse Lemur (<Emphasis Type="Italic">Microcebus sambiranensis</Emphasis>) in an Acoustic Survey of Habitat Preference

Primate vocalizations convey a variety of information to conspecifics. The acoustic traits of these vocalizations are an effective vocal fingerprint to discriminate between sibling species for taxonomic diagnosis. However, the vocal behavior of nocturnal primates has been poorly studied and there are few studies of their vocal repertoires. We compiled a vocal repertoire for the Endangered Sambirano mouse lemur, Microcebus sambiranensis, an unstudied nocturnal primate of northwestern Madagascar, and compared the acoustic properties of one of their call types to those of M. murinus and M. rufus. We recorded vocalizations from radio-collared individuals using handheld recorders over 3 months. We also conducted an acoustic survey to measure the vocal activity of M. sambiranensis in four forest habitat types at the study site. We identified and classified five vocalization types in M. sambiranensis. The vocal repertoires of the three Microcebus species contain very similar call types but have different acoustic properties, with one loud call type, the whistle, having significantly different acoustic properties between species. Our acoustic survey detected more calls of M. sambiranensis in secondary forest, riparian forest, and forest edge habitats, suggesting that individuals may prefer these habitat types over primary forest. Our results suggest interspecific differences in the vocal repertoire of mouse lemurs, and that these differences can be used to investigate habitat preference via acoustic surveys.     

The evolution of vocalisations in the genus Corvus: effects of phylogeny, morphology and habitat

In this study we investigated the phylogenetic, morphological and ecological factors affecting the caw calls of 28 species of the genus Corvus, spanning the worldwide range of the taxon. The three phylogeographic groups identified by Goodwin (1986, Crows of the World, British Museum (Natural History), St Edmudsbury Press, Bury St Edmunds), i.e. the American stock, the Palearctic-African stock and the Oriental-Australian stock, were differentiated by some of the acoustic features of their calls, suggesting that historical factors may have played an important role in the evolution of vocalisations in this group. To assess the effects of morphology (body size and bill length) and environment (open vs. closed habitat) and to simultaneously take into account the phylogenetic effects, we used the phylogenetically independent contrast method. This manner of analysis revealed that body size was important in shaping the acoustic attributes of the caw call, as it influenced two temporal and two spectral variables, whereas the effect of bill length was far weaker. Notably, our results did not confirm the negative correlation between call frequency and body size that resulted in a phylogeny-free analysis of the same data in many studies on passerines. Larger Corvus species, in fact, utter calls with higher fundamental frequency than those of smaller species. Hence, these results show that incorporating phylogeny in analyses can substantially alter the conclusions reached by studies carried out with non-phylogenetic approaches. The acoustic environment, considered one of the most important forces driving the evolution of vocalisations in passerines, slightly influenced only two acoustic parameters in the Corvusgenus, call fundamental frequency and duration of pulsed units, both of which increased in the calls of forest species.     

The effect of body size and habitat on the evolution of alarm vocalizations in rodents

When confronted with a predator, many mammalian species emit vocalizations known as alarm calls. Vocal structure variation results from the interactive effects of different selective pressures and constraints affecting their production, transmission, and detection. Body size is an important morphological constraint influencing the lowest frequencies that an organism can produce. The acoustic environment influences signal degradation; low frequencies should be favoured in dense forests compared to more open habitats (i.e. the ‘acoustic adaptation hypothesis’). Such hypotheses have been mainly examined in birds, whereas the proximate and ultimate factors affecting vocalizations in nonprimate mammals have received less attention. In the present study, we investigated the relationships between the frequency of alarm calls, body mass, and habitat in 65 species of rodents. Although we found the expected negative relationship between call frequency and body mass, we found no significant differences in acoustic characteristics between closed and open‐habitat species. The results of the present study show that the acoustic frequencies of alarm calls can provide reliable information about the size of a sender in this taxonomic group, although they generally do not support the acoustic adaptation hypothesis.     

RECENT BIOACOUSTIC PUBLICATIONS (2002 and earlier)

The vocal repertoire of the Asiatic Black Bear (Ursus thibetanus) has been poorly studied and until recently only two call types (chuffing and humming) have been described. Here I investigate the vocalizations of three wild orphaned cubs (two males and one female) reared by two observers in natural conditions in the Russian Far East. I grouped the calls into structural types, and then compared them with existing literature data on vocalizations of the same, as well as other species of the Ursidae family. In total, 1302 calls were classified visually from spectrograms into seven call types: whine, moan, yelp, grunt, snort, chuffing and humming. Classification results were verified with discriminant function analysis and randomization. I also fixed the presence of nonlinear phenomena (NLP) and articulation effects in calls. Whine was the most frequently recorded, as well as the most structurally variable call type due to a high rate of NLP. These results indicate that the vocal repertoire of the Asiatic Black Bear cubs is graded, but includes at least two discrete sound types. This work needs to be continued with further studies of vocalizations of cubs and adults of this species to verify the results of this preliminary study.     

Behavioural response evoked by conspecific distress calls in two neotropical treefrogs

Anurans emit distress calls when attacked by predators as a defensive mechanism. As distress calls may trigger antipredator behaviour even in individuals that are not under attack, we tested whether this defensive behaviour induced behavioural changes in neighbouring conspecifics. We compared the behavioural responses of two species of Neotropical hylid frogs (genus Boana) to conspecific distress calls and white noise. Individuals of both species interrupted their vocal activity and decreased call rate after hearing the distress call. Natural variation on signal intensity calibrated among the nearest neighbours did not influence the response and we did not observe negative phonotaxis after any acoustic stimulus. Despite the fact that many predators are acoustically oriented, we could not determine if such response (reduced call rate) was induced by risk assessment or by the masking effect on advertisement calls. Boana faber responded similarly to white noise and distress calls, while B. bischoffi responded more intensely to distress calls. Duration of silence after playbacks in B. faber was longer than B. bischoffi. We suggest that, if the signals are interpreted as a risk cue by neighbouring conspecifics, each species may be preyed upon by different predators, as they may have led to distinct defensive strategies and different responses to distress calls. If risk assessment information is included in distress calls, it triggers behavioural responses only in the nearest neighbours, as we did not observe responses on the vocal activity of the interspecific chorus. Our results add relevant data about acoustic communication and interpretations by anurans, highlighting the importance of considering cues within common and widespread signals.     

Acoustic Image of Common Rosefinch Carpodacus erythrinus (Pall.)

Acoustic analysis of the species-specific call of common rosefinch Carpodacus erythrinus was carried out. We compared calls of 12 individuals from the same habitat. The individual-specific elements were revealed in the calls. At the same time, species-specific elements were also recognized in all individuals. These elements compose the acoustic image of common rosefinch call including four syllables with intervals.