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ABSTRACTFlight calls, the quiet calls emitted by migratory birds on the wing, offer opportunities to understand the behaviour of birds during migration. We test the effectiveness of an eight-element microphone array for three-dimensional triangulation of the position of calling migratory birds. We constructed a microphone array out of commercially available components and used freely available software to process the recordings, so that this technology might be easily adopted for migration monitoring. In the Great Lakes region of North America, we triangulated the position of loudspeakers broadcasting synthetic tones and flight calls, as well as calls of actual passing migrants. Loudspeakers broadcasting synthetic tones showed a triangulation accuracy of 1.52 ± 0.34 m. Loudspeakers broadcasting flight calls of migratory wood-warblers showed a triangulation accuracy of 2.04 ± 0.37 m. Actual migratory warblers passing over the microphone array showed an estimated accuracy of 2.70 ± 0.48 m. We conclude that wireless microphone arrays accurately triangulate migrant birds, at least under optimal recording conditions. We present this proof-of-concept study to demonstrate the reliability of this underutilized technique which should be of interest as a tool for studying migratory bird behaviour, for quantifying migratory bird populations, and for monitoring the conservation of migratory birds.

ABSTRACTThe “zeep” complex consists of nine birds that produce nocturnal flight calls with similar acoustic features. Our inability to distinguish these calls inhibits the acoustic monitoring of these species. We test the hypothesis that flight calls of nine warblers in the “zeep” complex show sufficient acoustic differences to allow differentiation. We investigate divergence in these vocalizations by recording birds held for banding and collecting additional recordings from sound libraries. We used three approaches to compare calls between species: analysis of variance in acoustic properties, discriminant analysis of acoustic properties, and spectrographic cross-correlation. The first approach revealed five species that were different in one or more acoustic properties. The second approach revealed a level of assignment to the correct species (73%) that exceeded levels expected by chance (36%). The third approach revealed calls of seven species to be significantly more similar to conspecific calls than heterospecific calls. Our results suggest the calls of many members of the “zeep” complex exhibit species-specific differences in structure, which may allow differentiation of at least five “zeep” species based on call alone. We advocate for the combined use of these three approaches for the comparison of “zeep” calls in future flight call studies.

AbstractMany animals produce multiple types of breeding vocalizations that, together, constitute a vocal repertoire. In some species, the size of an individual’s repertoire is important because it correlates with brain size, territory size or social behaviour. Quantifying repertoire size is challenging because the long recordings needed to sample a repertoire comprehensively are difficult to obtain and analyse. The most basic quantification technique is simple enumeration, where one counts unique vocalization types until no new types are detected. Alternative techniques estimate repertoire size from subsamples, but these techniques are useful only if they are accurate. Using 12 years of acoustic data from a population of rufous-and-white wrens in Costa Rica, we used simple enumeration to measure the repertoire size for 40 males. We then compared these to the estimates generated by three estimation techniques: curve fitting, capture–recapture and a new technique based on the coupon collector’s problem. To understand how sampling effort affects the accuracy and precision of estimates, we applied each technique to six different-sized subsets of data per male. When averaged across subset sizes, the capture–recapture and coupon collector techniques showed the highest accuracy, whereas the curve fitting technique underestimated repertoire size. Precision (the average absolute difference between the estimated and true repertoire size) was significantly better for the capture–recapture technique than the coupon collector and curve fitting techniques. Both accuracy and precision improved as subset size increased. We conclude that capture–recapture is the best technique for estimating the sizes of small repertoires.

AbstractSong overlapping, a behaviour in which an individual begins singing before its counterpart has completed its song, has been the subject of recent debate. Although many studies have suggested that song overlapping functions as a signal, the majority of these studies fail to address the possibility that overlapping is a chance occurrence. Part of the difficulty in determining whether overlap is intentional or accidental lies in the lack of compelling null models for estimating chance levels of song overlap. We have developed the Song Overlap Null model Generator (SONG), a software package for R. SONG uses resampling randomization to predict the expected amount of overlap due to chance, and is applicable to any system in which individuals engage in signalling interactions. To evaluate the effectiveness of SONG, we examined the overlapping behaviour of three avian species: black-capped chickadees (Poecile atricapillus), rufous-and-white wrens (Thryophilus rufalbus) and long-tailed manakins (Chiroxiphia linearis). Our analyses revealed that black-capped chickadees avoided overlapping the songs of playback-simulated intruders, duetting wrens overlapped the songs of their mates and manakins avoided overlapping the duets of their neighbours. We believe that SONG will prove to be a valuable tool for understanding signal timing in songbirds as well as other taxa.

The Acoustic Adaptation Hypothesis posits that habitat characteristics influence the structure of animal vocalizations and that animals will vocalize and display behaviours optimized for sound transmission. White-eared ground-sparrows Melozone leucotislive in habitats with dense vegetation where vocal communication is an ideal mode of communication for territory defence and mate attraction. On the basis of the Acoustic Adaptation Hypothesis, if solos and duets of these ground-sparrows are used in long-distance communication, we should expect that these vocalizations will exhibit structures that enhance sound transmission. We conducted a sound transmission experiment where we broadcast and re-recorded solo songs and duets to study their transmission properties. We used two speaker heights and two microphone heights to simulate different perch heights of signallers and receivers and four distances between the speakers and microphones to simulate variable distances of separation. We found that solo and duet songs show similar patterns of degradation and attenuation with distance and proximity to the ground. These results suggest that solo and duet songs facilitate communication with receivers at similar distances. The highest perches, for both signallers and receivers, maximized acoustic transmission. This is the first study that evaluates the transmission properties of songs and duets in birds, despite the fact that many bird species in tropical forests produce both types of vocalizations. To our surprise, we found that solo and duet songs degraded to below-detectable levels in less than a typical territory's diameter, suggesting that this species has not experienced strong selection for long-distance communication.

Acoustic localization is a powerful technique for monitoring the positions, movements and behaviours of terrestrial animals. However, its prevalence in biological studies has been constrained by hardware and software that are custom-built, expensive and difficult to use. We recently helped to relieve the hardware constraint by describing a microphone array that is affordable, portable, easy to use and commercially available. Here, we help to relieve the software constraint by developing an acoustic localization program called “Sound Finder”, which is easy to use, freely available and accurate for a variety of animals and recording conditions. It runs in the free software environment R, and in spreadsheet programs such as Microsoft Excel and the open-source software LibreOffice. In this study, we describe how Sound Finder functions, and then test its accuracy by localizing natural sounds that were broadcast through loudspeakers and re-recorded with microphone arrays. We quantify Sound Finder's accuracy by comparing its location estimates with known loudspeaker locations and with output from other localization approaches. We show that Sound Finder generates accurate location estimates for a variety of animal sounds, microphone array configurations and environmental conditions. Furthermore, Sound Finder generates an error value that allows the user to assess its accuracy. In conclusion, Sound Finder provides accurate estimates of a vocalizing animal's location. It is easy to use, requires only widespread and affordable software and is freely available in a standard form as Supplementary material to this article.

Vocal communication in duetting and chorusing birds is a growing area of study in avian ecology, yet much remains unknown about temporal and population-level variation in these complex vocal signals. In this study, we describe the acoustic structure and temporal variation in solos, duets, and choruses in the Rufous-naped Wren (Campylorhynchus rufinucha), a cooperatively breeding neotropical passerine. We collected focal recordings of 19 groups to assess both diel and seasonal variation in vocal output, as well as population-wide sharing of vocal signals. We found that birds produce a complex array of vocalisations, including tonal, frequency-modulated syllables grouped into phrases, as well as stereotyped, atonal sounds. Songs are produced as solos or combined into duets and choruses. Solo and duet songs show a dawn chorus effect. Solo song rate, but not duet or chorus rate, varied across breeding stages. The majority of phrases are shared amongst groups, significantly more amongst groups in nearby territories. We suggest that chorus songs may be an important indicator of group identity and may play a role in maintaining group territories, but do not play a role in relation to the breeding cycle. The degree of population-wide phrase-sharing suggests either short-distance dispersal or delayed song learning. This paper is the first fine-scale description of vocal behaviour in this species and enhances our understanding of group-singing in a complex social environment.

ABSTRACTSyrinx is a newly developed PC-based software package designed for interactive playback of animal vocalizations and for digital sound recording and analysis in the lab or in the field. Syrinx permits flexible modeling of complex vocal interactions in an interactive playback setting. Large numbers of sounds can be stored on disk as digital wave files for instant recall in the field. The playback trial timer and real-time scrolling spectrographs display of microphone input facilitate the tailoring of playback responses to subtle changes in the vocalizations of playback subjects. Based on our field test of this software in an interactive playback experiment with Black-capped Chickadees Poecile atricapillus, we conclude that Syrinx is a versatile and user-friendly tool for performing interactive playback with a minimum of equipment.

Many territorial animals exhibit reduced aggression towards neighbours. Known as “the dear enemy effect”, this phenomenon has been documented among conspecific animals across a wide range of animal taxa. In theory, the dear enemy effect can also exist between individuals of different species, particularly when those species compete for shared resources. To date, a heterospecific dear enemy effects has only been documented in ants. In this study, we test for both a conspecific and heterospe-cific dear enemy effect in neotropical rufous-and-white wrens Thryophilus rufalbus. This species competes for resources with banded wrens Thryophilus pleurostictus, a closely related sympatric congener. We used acoustic playback to simulate ruf-ous-and-white wren and banded wren neighbours and non-neighbours at the edges of rufous-and-white wren territories. Ruf-ous-and-white wrens responded more strongly to signals from their own species, demonstrating that resident males discriminate between conspecific and heterospecific rivals. They did not, however, exhibit a conspecific dear enemy effect. Further, they did not exhibit a heterospecific dear enemy effect. This could be due to neighbours and non-neighbours posing similar levels of threat in this system, to the possibility that playback from the edges of the subjects’ large territories did not simulate a threatening signal, or to other factors. Our study provides the first test of a heterospecific dear enemy effect in vertebrates, and presents a valuable experimental approach for testing for a heterospecific dear enemy effect in other animals.

Acoustic interference can impede effective communication that is important for survival and reproduction of animals. In response to acoustic interference, some animals can improve signalling efficacy by altering the structure of their signals. In this study, we played artificial noise to 46 male spring peepers Pseudacris crucifer, on their breeding grounds, and tested whether the noise affected the duration, call rate, and peak frequency of their advertisement calls. We used two experimental noise treatments that masked either the high- or low-frequency components of an average advertisement call; this allowed us to evaluate whether frogs adaptively shift the peak frequency of their calls away from both types of interference. Our playback treatments caused spring peepers to produce shorter calls, and the high-frequency noise treatment caused them to lower the frequency of their calls immediately after the noise ceased. Call rate did not change in response to playback. Consistent with previous studies, ambient temperature was inversely related to call duration and positively related to call rate. We conclude that noise affects the structure of spring peeper advertisement calls, and that spring peepers therefore have a mechanism for altering signal structure in response to noise. Future studies should test if other types of noise, such as biotic or anthropogenic noise, have similar effects on call structure, and if the observed changes to call structure enhance or impair communication in noisy environments [Current Zoology 60 (4): 438–448, 2014].