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Density estimates accounting for differential animal detectability are difficult to acquire for wide-ranging and elusive species such as mammalian carnivores. Pairing distance sampling with call-response surveys may provide an efficient means of tracking changes in populations of coyotes (Canis latrans), a species of particular interest in the eastern United States. Blind field trials in rural New York State indicated 119-m linear error for triangulated coyote calls, and a 1.8-km distance threshold for call detectability, which was sufficient to estimate a detection function with precision using distance sampling. We conducted statewide road-based surveys with sampling locations spaced ≥6 km apart from June to August 2010. Each detected call (be it a single or group) counted as a single object, representing 1 territorial pair, because of uncertainty in the number of vocalizing animals. From 524 survey points and 75 detections, we estimated the probability of detecting a calling coyote to be 0.17 ± 0.02 SE, yielding a detection-corrected index of 0.75 pairs/10 km2 (95% CI: 0.52–1.1, 18.5% CV) for a minimum of 8,133 pairs across rural New York State. Importantly, we consider this an index rather than true estimate of abundance given the unknown probability of coyote availability for detection during our surveys. Even so, pairing distance sampling with call-response surveys provided a novel, efficient, and noninvasive means of monitoring populations of wide-ranging and elusive, albeit reliably vocal, mammalian carnivores. Our approach offers an effective new means of tracking species like coyotes, one that is readily extendable to other species and geographic extents, provided key assumptions of distance sampling are met. © 2015 The Wildlife Society.

Autonomous sound recording techniques have gained considerable traction in the last decade, but the question remains whether they can replace human observation surveys to sample sonant animals. For birds in particular, survey methods have been tested extensively using point counts and sound recording surveys. Here, we review the latest evidence for this taxon within the frame of a systematic map. We compare sampling effectiveness of these two survey methods, the output variables they produce, and their practicality. When assessed against the standard of point counts, autonomous sound recording proves to be a powerful tool that samples at least as many species. This technology can monitor birds in an exhaustive, standardized, and verifiable way. Moreover, sound recorders give access to entire soundscapes from which new data types can be derived (vocal activity, acoustic indices). Variables such as abundance, density, occupancy, or species richness can be obtained to yield data sets that are comparable to and compatible with point counts. Finally, autonomous sound recorders allow investigations at high temporal and spatial resolution and coverage, which are more cost effective and cannot be achieved by human observations alone, even though small-scale studies might be more cost effective when carried out with point counts. Sound recorders can be deployed in many places, they are more scalable and reliable, making them the better choice for bird surveys in an increasingly data-driven time. We provide an overview of currently available recorders and discuss their specifications to guide future study designs.

Territorial responses by North American red squirrels ( Tamiasciurus hudsonicus ) to conspecifics vary seasonally with peaks during mating and dispersal periods. Broadcast of squirrel vocalizations during surveys may elicit territorial defense behaviors such as calling and movement that make individuals more available for detection, with implications for subsequent occupancy and abundance analyses. We examined the effect of vocalization broadcasts on detection probability during point counts throughout a 14-month period at two locations (year-round study) and during two summers at a third location (summer-only study) on Newfoundland, Canada. Overall detection probability based on sight and sound varied seasonally but the use of vocalization broadcast consistently enhanced detection probability. Squirrels were also more likely to be seen during vocalization broadcast survey periods than during silent point counts. Response to vocalization broadcast was highest when local population density was lowest. Higher detection probability during the initial silent periods of our surveys, when population density was high, likely reflects the greater chance of spontaneous vocalizations in response to the behavior of neighbors.

1. Autonomous sound recording is a promising sampling method for birds and other vocalizing terrestrial wildlife. However, while there are clear advantages of passive acoustic monitoring methods over classical point counts conducted by humans, it has been difficult to quantitatively assess how they compare in their sampling performance. Quantitative comparisons of species richness between acoustic recorders and human point counts in bird surveys have previously been hampered by the differing and often unknown detection ranges or sound detection spaces among sampling methods. 2. We performed two meta-analyses based on 28 studies where bird point counts were paired with sound recordings at the same sampling sites. We compared alpha and gamma richness estimated by both survey methods after equalizing their effective detection ranges. We further assessed the influence of technical sound recording specifications (microphone signal-to-noise ratio, height and number) on the bird sampling performance of sound recorders compared to unlimited radius point counts. 3. We show that after standardizing detection ranges, alpha and gamma richness from both methods are statistically indistinguishable, while there might be an avoidance effect in point counts. Furthermore, we show that microphone signalto-noise ratio (a measure of its quality), height and number positively affect performance through increasing the detection range, allowing sound recorders to match the performance of human point counts. 4. Synthesis and applications. We demonstrate that when used properly, high-end sound recording systems can sample terrestrial wildlife just as well as human observers conducting point counts. Correspondingly, we suggest a first standard methodology for sampling birds with autonomous sound recorders to obtain results comparable to point counts and enable practical sampling. We also give recommendations for carrying out effective surveys and making the most out of autonomous sound recorders.

The Vitelline Warbler ( Setophaga vitellina ) is an understudied species endemic to a few small islands in the western Caribbean. Little is known beyond its phylogenetic relationship to other New World warblers. We used island-wide surveys and bioacoustic recordings to investigate the distribution, vocalizations, and ecology of S. vitellina across a significant portion of the species’ range on Little Cayman Island. We recorded 417 songs from 91 individuals and analyzed the length, frequency, and shape of various song components. We observed and characterized high variation in the composition and character of songs within the Little Cayman population. We also describe the call of the species and use sound files from across the species’ range to compare vocalizations between islands. Vitelline Warbler abundance is highest in dry forest and dry scrub habitats, suggesting that these habitats are most important for the species. Elaboration of the vocalizations of understudied species like the Vitelline Warbler has the potential to further our understanding of avian evolution and behavior. As much still remains to be learned from this species, action must be taken to protect its critical habitats, especially dry forests, among other conservation measures.

Bias introduced by detection errors is a well-documented issue for abundance and occupancy estimates of wildlife. Detection errors bias estimates of detection and abundance or occupancy in positive and negative directions, which can produce misleading results. There have been considerable design- and model-based methods to address false-negative errors, or missed detections. However, false-positive errors, or detections of individuals that are absent but counted as present because of misidentifications or double counts, are often assumed to not occur in ecological studies. The dependent double-observer survey method is a design-based approach speculated to reduce false positives because observations have the ability to be confirmed by two observers. However, whether this method reduces false positives compared to single-observer methods has not been empirically tested. We used prairie songbirds as a model system to test if a dependent double-observer method reduced false positives compared to a single-observer method. We used vocalizations of ten species to create auditory simulations and used naive and expert observers to survey these simulations using single-observer and dependent double-observer methods. False-positive rates were significantly lower using the dependent double-observer survey method in both observer groups. Expert observers reported a 3.2% false-positive rate in dependent double-observer surveys and a 9.5% false-positive rate in single-observer surveys, while naive observers reported a 39.1% false-positive rate in dependent double-observer surveys and a 49.1% false-positive rate in single-observer surveys. Misidentification errors arose in all survey scenarios and almost all species combinations. However, expert observers using the dependent double-observer method performed significantly better than other survey scenarios. Given the use of double-observer methods and the accumulating evidence that false positives occur in many survey methods across different taxa, this study is an important step forward in acknowledging and addressing false positives.

Livestock grazing is an important management tool for biodiversity conservation in many native grasslands across the globe. Understanding how different grazing species interact with their environment is integral to achieving conservation goals. In the semiarid grasslands of Australia, grazing by sheep or cattle is used to manipulate vegetation structure to suit the habitat needs of a globally unique, critically endangered grassland bird, the plains-wanderer Pedionomus torquatus. However, there has been no investigation of whether sheep and cattle differ in their effects on plains-wanderer habitat and, therefore, it is unknown if these grazers are substitutable as a management tool. Using a grazing experiment in native grasslands over 3 years, we determined the effects of grazer type (sheep, cattle) on occurrence and vocal activity of plains-wanderer, vegetation structure and composition, and food availability. We also examined grazer effects on encounter rates of other grassland birds. Plains-wanderer breeding activity was inferred from vocalization rates captured by bioacoustic recorders. Spotlighting was used to measure encounter rates of other grassland birds. We found that different grazers altered the structure of the habitat. Grasslands grazed by cattle were typically more open, less variable, and lacked patches of dense vegetation relative to those grazed by sheep. Grazer type did not influence the likelihood of plains-wanderer occurrence, but it did interact with year of survey to affect breeding activity. The number of days with one or more calls significantly increased at sheep grazed sites in year-3, which coincided with enduring drought conditions. Similarly, grazer effects on encounter rate of all birds, bird species richness, and Australasian pipit Anthus novaeseelandiae were different between years. Dense vegetation specialists (such as stubble quail Coturnix pectoralis) were positively associated with grasslands grazed by sheep. As a habitat management tool, sheep or cattle grazing are useful when the goal is to support an open grassland structure for the plains-wanderer. However, their substitutability is likely to be dependent upon climate. We caution that a loss of dense vegetation in grasslands grazed by cattle during drought could limit the availability of optimal habitat for the plains-wanderer and habitat for other grassland birds.

Noise may drive changes in the composition and abundance of animals that communicate vocally. Traffic produces low-frequency noise (<3 kHz) that can mask acoustic signals broadcast within the same frequency range. We evaluated whether birds that sing within the frequency range of traffic noise are affected by acoustic masking (i.e., increased background noise levels at the same frequency of vocalizations reduce detection of vocalization) and are less abundant in areas where traffic noise is loud (44-57 dB). We estimated occupancy, the expected probability that a given site is occupied by a species, and detection probabilities of eight forest-breeding birds in areas with and without traffic noise as a function of noise and three measures of habitat quality: percent forest cover, distance from plot center to the edge of forest, and the number of standing dead trees or snags. For the two species that vocalize at the lowest peak frequency (the frequency with the most energy) and the lowest overall frequency (Yellow-billed Cuckoo [Coccyzus americanus] and White-breasted Nuthatch [Sitta carolinensis]), the presence of traffic noise explained the greatest proportion of variance in occupancy, and these species were 10 times less likely to be found in noisy than in quiet plots. For species that had only portions of their vocalizations overlapped by traffic noise, either forest cover or distance to forest edge explained more variation in occupancy than noise or no single variable explained occupancy. Our results suggest that the effects of traffic noise may be especially pronounced for species that vocalize at low frequencies. El ruido puede producir cambios en la composición y abundancia de animales que se comunican vocalmente. El tráfico produce ruido de baja frecuencia (<3 kHz) que puede enmascarar la transmisión de señales acústicas en el mismo rango de frecuencia. Evaluamos sí las aves que cantan en el rango de frecuencia del ruido de tráfico son afectadas por el enmascaramiento acústico (i.e., el incremento en los niveles de ruido de fondo con la misma frecuencia de las vocalizaciones reduce la detección de vocalización) y son menos abundantes en áreas donde el ruido de tráfico es alto (44-57 dB). Estimamos la ocupación, la probabilidad esperada de que un sitio determinado sea ocupado por una especie, y las probabilidades de detección de 8 especies de aves de bosque en áreas con y sin ruido de tráfico como una función del ruido y 3 medidas de calidad de hábitat: porcentaje de cobertura forestal, distancia del centro de la parcela al borde del bosque y el número de árboles muertos en pie. Para las especies que vocalizan en el pico menor de frecuencia (la frecuencia con mayor energía) y en la menor frecuencia total (Coccyzus americanus y Sitta carolinensis), la presencia de ruido de tráfico explicó la mayor proporción de varianza en la ocupación, y fue 10 veces menos probable encontrar estas especies en parcelas ruidosas que en parcelas silenciosas. Para especies cuyas vocalizaciones se traslaparon parcialmente con el ruido de tráfico, la variación en la ocupación fue explicada más por la cobertura vegetal o la distancia al borde del bosque que por el ruido o ninguna variable individual explicó la ocupación. Nuestros resultados sugieren que los efectos del ruido de tráfico pueden ser especialmente pronunciados para especies que vocalizan en frecuencias bajas.

Previous studies have posited the use of acoustics-based surveys to monitor population size and estimate their density. However, decreasing the bias in population estimations, such as by using Capture–Mark–Recapture, requires the identification of individuals using supervised classification methods, especially for sparsely populated species like the wolf which may otherwise be counted repeatedly. The cryptic behaviour of Indian wolf ( Canis lupus pallipes ) poses serious challenges to survey efforts, and thus, there is no reliable estimate of their population despite a prominent role in the ecosystem. Like other wolves, Indian wolves produce howls that can be detected over distances of more than 6 km, making them ideal candidates for acoustic surveys. Here, we explore the use of a supervised classifier to identify unknown individuals. We trained a supervised Agglomerative Nesting hierarchical clustering (AGNES) model using 49 howls from five Indian wolves and achieved 98% individual identification accuracy. We tested our model’s predictive power using 20 novel howls from a further four individuals (test dataset) and resulted in 75% accuracy in classifying howls to individuals. The model can reduce bias in population estimations using Capture-Mark-Recapture and track individual wolves non-invasively by their howls. This has potential for studies of wolves’ territory use, pack composition, and reproductive behaviour. Our method can potentially be adapted for other species with individually distinctive vocalisations, representing an advanced tool for individual-level monitoring.

The Charleston Harbor in South Carolina (SC)is a major port that experiences high levels of vessel traffic. Historical analyses of coastal bottlenose dolphin (Tursiops truncatus, now Tursiops erebennus) sightings identified multiple core use areas in the harbor that overlap with these anthropogenic activities. Informed by these long-term spatial data, passive acoustic monitoring, visual surveys, and prey sampling were conducted from December 2017 to June 2019 to assess the relationships and multivariate interactions that may influence dolphin vocalization patterns. Vocalizations varied spatially and temporally, peaking in fall and winter months coinciding with decreases in water temperature and daylight hours, following patterns previously reported in other SC estuaries. Dolphin prey and total fish abundance decreased with water temperature, which may indicate that dolphins echolocate and whistle more frequently in the winter months when prey are scarce and sound-producing species are less soniferous. Dolphin sightings and vocalizations were highly correlated. Dolphin occurrence was highest in the areas surrounding the confluence of the Cooper and Wando Rivers, along the shipping channel, where vessel and sound-producing fish detections were greatest. When vessel noise occurred, dolphins increased their vocalizations, which suggests that this population may be modifying its acoustic repertoire in response to increased noise levels. Multivariate interactions indicate strong spatial and seasonal patterns in vocalization rates that may be associated with dolphin and prey abundance as well as noise-induced redundancy.