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Among territorial birds, the number of conspecific neighbours sets the social context of communication. There have been many investigations of vocal behaviour and its important role in territory defense and mate attraction in birds; however, the effect of the density of conspecific neighbours on avian communication has received little attention. In this study, we reviewed the literature on the influence of local density on vocal communication in birds, exploring how the number of conspecific neighbours influences avian vocal behaviour, by conducting a literature review following the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) in two databases: Web of Science and Scopus. We also included additional articles from a more flexible search in Google Scholar. We found 19 studies of birds that included data on vocal behaviour in relation to the number of conspecific neighbours. The majority of these studies (14 of 19) showed that local density influences avian vocal behaviour. Overall, these studies revealed an effect of local density of conspecifics on song rate (expressed as song rate, syllable rate, solo song rate, or singing rate). Other vocal behaviours that were shown to vary with local density included song-type switching, song duration, peak frequency, and song variation. Most studies focused on temperate-zone species where males are the predominant singers. Our results reveal a bias towards the study of male songs in the context of local density, with very few studies on female song; this corresponds with a historical geographical focus on temperate birds. We present experimental design suggestions for future investigations, including predictions for males and females of tropical and temperate species. Finally, we argue that more research on this topic is needed, and that investigations of vocal communication will benefit from further study of the influence of density on avian vocal behaviour. [ABSTRACT FROM AUTHOR]

Elaborate visual signals play a crucial role in communication between many breeding animals, often evolving through female mate choice and male-male competition. Neotropical Yellow Toads (Incilius luetkenii) exhibit a temporary elaborate visual trait during their brief annual mating events, transitioning from a drab, female-like brown coloration to a vibrant lemon-yellow. They engage in scramble competition, where males physically compete with both lone and amplectant males for access to females. Previous studies suggest that male color may serve as a sex identification signal in the toads' large breeding aggregations, and that females do not appear to choose males based on color. In this study we investigated whether variation in male coloration functions as an intrasexual signal during male-male interactions. Using realistic robotic model toads in a two-choice arena, we addressed two questions. First, do male Yellow Toads respond differently to a lone male versus a male in amplexus with a female when both males are the same color? Secondly, do male Yellow Toads respond differently to two amplectant males varying in the brightness of their yellow coloration? Males approached the amplectant pair nearly twice as often as the lone male, although this pattern was not statistically significant. Males did not exhibit a differential reaction to bright amplectant males versus dull amplectant males. Therefore, variation in male yellow color and the presence of a female do not appear to influence male-male competitive behavior. Overall, this study contributes to our understanding of temporary color change in vibrantly colored anuran species. [ABSTRACT FROM AUTHOR]

Animals use colouration to serve diverse functions including camouflage, thermoregulation, and communication. Recent research has revealed that many anurans exhibit drastic colour changes and growing evidence supports that these changes are sexually selected signals. Male yellow toads, Incilius luetkenii , exhibit dynamic sexual dichromatism, changing from mud-brown to lemon-yellow during their brief breeding events. Toads darken when isolated in captivity, which is hypothesized to be a stress response, although the mechanisms driving this change have yet to be experimentally investigated. We confined breeding toads to small terrariums for four hours and predicted that colour and corticosterone levels would change in isolation. We found that toads darkened during isolation, but that corticosterone levels did not change with colour. Our correlational results suggest that corticosterone is not the main driver of colour change in yellow toads and highlight avenues for future research that may enhance our understanding of colour change in anurans. [ABSTRACT FROM AUTHOR]

Eavesdropping on interactions between conspecific animals provides a low‐cost method for assessing other individuals. Asymmetries in territorial counter‐singing interactions in songbirds provide a rich source of information for eavesdroppers about differences between the singers. Yet, little is known about the relationship between interactive singing in a natural, low‐arousal context among territorial neighbours and individual traits of males. We used a microphone array to monitor natural counter‐singing interactions in great tits (Parus major) during nest building, at the onset of the breeding season. We quantified song overlapping and song matching for 30 pairs (dyads) of interacting males, singing at their nest, respectively. We then compared these behaviours to five traits for 28 males: body condition, plumage ornamentation, offspring provisioning behaviour, offspring weight and breeding site quality. We found no relationship between a male song overlapping or matching behaviour and any of the measured traits. Therefore, our results do not support the idea that short‐term asymmetries in low‐arousal long‐range singing interactions among neighbours reflect differences in these fitness‐related traits. Instead, our findings suggest that such singing asymmetries have less signal value in the absence of an immediate conflict but instead reflect short‐term motivational differences, as shown in previous investigations. [ABSTRACT FROM AUTHOR]

Animals that migrate in mixed-species groups may communicate with both conspecific and heterospecific individuals, providing a low-cost mechanism for navigation whenever individuals share similar migratory routes or destinations. Many migratory birds produce calls while flying, but the function of these calls, and the forces contributing to their evolution, are poorly known. We studied flight calls in mixed-species groups of wood warblers (Parulidae), a biodiverse group of migratory songbirds. We used a spatial approach to examine whether acoustic similarity of flight calls varies with group composition, recording flight calls of mixed-species flocks with a wireless microphone array and triangulating the positions of birds in three dimensions. We found that the acoustic similarity of flight calls was correlated with spatial proximity: birds with similar calls fly closer together during migration. We also found relationships between acoustic similarity, flock size and mixed-species flock diversity: birds with similar calls fly in smaller flocks and in flocks with lower species diversity. Our results support the idea that migrating birds use flight calls to maintain contact with acoustically similar individuals in mixed-species flocks, with communication transcending species boundaries. These results suggest that acoustically similar flight calls are used as cues of group assembly for migratory animals. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'. [ABSTRACT FROM AUTHOR]

Conspicuous visual signals play an important role in animal communication, both within and between species. Bright, colourful signals allow animals to discriminate between individuals and can inform behavioural decisions and social interactions. In many instances, conspicuous colouration appears to have evolved through sexual selection via female mate choice or male–male competition. Male Neotropical yellow toads, Incilius luetkenii, display bright‐yellow colouration during their brief explosive mating events and then revert to a cryptic, female‐like, brown colour following amplexus and fertilization. Recent research has revealed that the yellow colouration of males may serve as a sex identification signal and that males remain yellow in the presence of both males and females but darken without conspecific stimulus. However, there is considerable variation in the brightness of yellow colouration in courting males, ranging from dull olive‐green to vibrant lemon‐yellow. The function of this variation remains unknown. In this study, we conducted two‐choice model presentation trials to test one mechanism of sexual selection that may drive this variation: intersexual selection. Specifically, we set out to determine whether females differentiate between males based on their colouration. We presented females with two hyper‐realistic robotic model toads, with one model painted to match a bright‐yellow male and the other a dull‐yellow male. Using several metrics of choice, we found that females did not show a preference for bright or dull colouration. Our research suggests that variation in the intensity of male breeding colouration is not driven by female choice in this species. Our study is consistent with the idea that there may be limited opportunity for female choice to influence male‐trait expression in explosive breeders. Future research will address whether intrasexual selection may influence colour variation in male yellow toads. [ABSTRACT FROM AUTHOR]

Flight 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. [ABSTRACT FROM AUTHOR]

Animals often live within close proximity of multiple conspecific individuals, allowing them to eavesdrop on other animals' signalling interactions to guide their own social behaviours. For a young animal that is learning to vocalize, eavesdropping on vocal interactions between adults may provide a rich source of information: young animals might preferentially learn vocalizations that are commonly heard in interactions between adults or that are heard to be effective for attracting mates or defending resources. We used a multispeaker playback experiment with wild Savannah sparrows, Passerculus sandwichensis , to test the hypothesis that vocal learning is guided by eavesdropping. Over a 6-year period, we tutored young Savannah sparrows with experimental tutor songs; half of the tutor songs were broadcast in simulated vocal interactions between two tutors, and the other half were broadcast as noninteractive, stand-alone solo performances. If eavesdropping plays an important role in guiding vocal learning, we predicted that young birds would preferentially learn the vocalizations heard during interactions between tutors. In contrast to our prediction, young Savannah sparrows did not preferentially learn interactive tutor songs; birds were similarly likely to learn songs heard in an interactive context (N = 13) and in a noninteractive context (N = 17). Analysis of live adult tutors' reactions to the loudspeakers showed that they responded with similar vocal behaviour during interactive and noninteractive treatments, and therefore their vocal behaviour did not compromise the playback simulation. We conclude that eavesdropping on vocal interactions between tutors does not appear to be essential for vocal learning of wild Savannah sparrows. • Animals that learn vocalizations may eavesdrop on interactions between tutors. • We used playback to study learning through eavesdropping in wild Savannah sparrows. • Birds learned songs at similar rates from interactive and noninteractive tutors. • Live tutors did not sing differently with interactive versus noninteractive tutors. • We found no evidence that eavesdropping guides vocal learning in Savannah sparrows. [ABSTRACT FROM AUTHOR]

Interspecific territoriality is frequently reported between closely related species; however, few studies have demonstrated interspecific territoriality between distantly related species living in sympatry. We conducted playback experiments to investigate territorial behaviour in male and female White-bellied Wrens ( Uropsila leucogastra) in response to simulated conspecific and heterospecific intruders during the breeding and non-breeding seasons. We explored whether heterospecific songs of the Happy Wren ( Pheugopedius felix), a distantly related species and ecological competitor, elicited antagonistic responses from focal White-bellied Wrens, and whether such responses differed between the sexes. We also examined whether male and female responses to conspecific and heterospecific rivals varied with season. We found that male White-bellied Wrens always responded to conspecific song, and responded significantly more to heterospecific song compared to a control stimulus (Tropical Parula, Setophaga pitiayumi). In contrast, although female White-bellied Wrens responded strongly to conspecific song, their response to heterospecific song did not differ significantly from the control stimulus. The proportion of males that responded to heterospecific songs and the proportion of females that responded to conspecific songs varied seasonally, showing significantly lower responses during the breeding season. The intense responses of male White-bellied Wrens to playback of heterospecific songs suggest that they recognise ecological competitors based on their vocal signals. Furthermore, the decrease in agonistic interactions during the breeding season is in line with the hypothesis that aggressive behaviour may be detrimental to reproductive and parental activity, and the hypothesis that heterospecific animals pose less of a threat during the breeding season. [ABSTRACT FROM AUTHOR]

Many territorial animals are less aggressive towards neighbours than they are towards strangers. This phenomenon is known as the ‘dear enemy’ effect and it occurs because strangers represent a considerably higher threat to territory take-over compared to neighbours. Some evidence has suggested that large repertoires may constrain neighbour–stranger discrimination. We tested whether songbirds with large repertoires exhibit neighbour–stranger discrimination, conducting a playback study on a songbird with a large vocal repertoire, and a comparative analysis of the dear enemy effect across all published studies of songbirds. In our playback study, we broadcast neighbour and stranger songs within the breeding territories of red-eyed vireos, Vireo olivaceus , a songbird species with a large song repertoire (ca. 50 songs per individual). Vireos responded significantly more aggressively to playback of stranger versus neighbour songs; subjects approached closer to the loudspeaker, had a lower latency to approach the loudspeaker, spent more time near the loudspeaker and sang more soft songs during stranger trials than during neighbour trials. We examined song sharing between red-eyed vireos and found low levels of song sharing between neighbours, suggesting that red-eyed vireos may discriminate among conspecifics based on individually distinctive song types. We then conducted a comparative analysis of neighbour–stranger discrimination across the published literature on songbirds, using a phylogenetically controlled analysis to explore whether species with large repertoires are less likely to discriminate between neighbours and strangers. Across 34 species, we found no evidence that songbirds with large repertoires are constrained in their ability to distinguish between neighbours and strangers. We conclude that large song repertoires do not inhibit neighbour–stranger discrimination in red-eyed vireos specifically, or songbirds generally. [ABSTRACT FROM AUTHOR]

Many animals breed in large mating aggregations, where males must rapidly discriminate between prospective mates and rivals. Selection may favour features that facilitate rapid discrimination in these aggregations. The explosively breeding Neotropical Yellow Toad, Incilius luetkenii , exhibits a rapid and dramatic colour change; males change from a cryptic brown to a conspicuous lemon yellow for their brief breeding period. Females, in contrast, remain cryptic brown throughout the year. The function of this temporary, sex-specific colour change is unknown. We tested the hypothesis that yellow colouration displayed by male I. luetkenii facilitates sex recognition during both daytime and nighttime mating aggregations. We created yellow and brown model toads and presented them to males during a breeding event. Male I. luetkenii responded significantly more intensely to brown models compared to yellow models, approaching them and making more amplexus attempts on the brown versus yellow models. This strong pattern held true regardless of ambient light intensity, making this the first study to expose a dynamic colour signal that operates during both day and night. Our results indicate that male I. luetkenii use colouration to quickly discriminate between males and females during their brief, explosive mating aggregations. Our findings suggest that the rapid, dramatic colour change of male I. luetkenii facilitates sex recognition, which could provide a significant fitness advantage to males in the form of reduced energy expenditure and reduced risk of injury by other males. Dynamic dichromatism may provide similar fitness benefits in any organisms that mate in large, competitive aggregations. [ABSTRACT FROM AUTHOR]

Many birds and primates use loud vocalizations to mediate agonistic interactions with conspecifics, either as solos by males or females, or as coordinated duets. The extensive variation in duet complexity, the contribution of each sex, and the context in which duets are produced suggest that duets may serve several functions, including territory and mate defense. Titi monkeys ( Callicebus spp.) are believed to defend their home range via solo loud calls or coordinated duets. Yet there are remarkably few experimental studies assessing the function of these calls. Observations of interactions between wild established groups and solitary individuals are rare and, therefore, controlled experiments are required to simulate such situations and evaluate the mate and joint territorial defense hypotheses. We conducted playback experiments with three free-ranging groups of habituated black-fronted titi monkeys ( Callicebus nigrifrons) to test these hypotheses. We found that titi monkeys responded to the three conspecific playback treatments (duets, female solos, and male solos) and did not respond to the heterospecific control treatment. The monkeys did not show sex-specific responses to solos ( N = 12 trials). Partners started to duet together in 79% of their responses to playback-simulated rivals ( N = 14 calls in response to playback). Males started to approach the loudspeaker before females regardless of the type of stimulus. The strength of the response of mated pairs to all three conspecific treatments was similar. Overall, our results are consistent with the idea that black-fronted titi monkeys use their loud calls in intergroup communication as a mechanism of joint territorial defense. Am. J. Primatol. 77:1135-1142, 2015. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Vertebrate vocalizations are widespread secondary sexual signals used for mate attraction and territory defence, and variation in signal quality is often condition dependent and impacts reproductive outcomes. Although vocal signal performance is known to reflect various aspects of male quality, few studies have examined the underlying mechanisms mediating its costs and hence its honesty. Using a population of Arctic-breeding snow buntings ( Plectrophenax nivalis), we compared the ' Oxidation Handicap Hypothesis', which predicts that testosterone-induced increases in oxidative stress provide a direct mechanistic basis for ensuring the honesty of many secondary sexual signals, to the ' Aerobic Activity Hypothesis, which predicts that it is the aerobic activity involved with signal production (i.e. vocal performance or defending a large territory) and not testosterone directly that links signal quality and oxidative stress. Males singing at faster rates had higher levels of both reactive oxygen metabolites and non-enzymatic antioxidant capacity in the plasma (i.e. without an increase in overall oxidative stress), enabling certain males to produce high-quality signals while also mitigating the costs of an associated increase in oxidative stress. However, these results were completely independent of plasma testosterone levels, supporting the role of aerobic performance in directly affecting oxidative stress. Although song performance was not linked to reproductive parameters in our data set, our research is the first to test these competing hypotheses in a behavioural trait and results suggest that oxidative stress may be an underlying physiological cost preventing low-quality individuals from producing high-quality signals. [ABSTRACT FROM AUTHOR]

Loud calls can be heard over long distances due to their high amplitude and low frequency. These calls are commonly used for both within- and between-group communication in many bird and primate species. In the Neotropics, mated pairs of socially monogamous titi monkeys (genus Callicebus) emit conspicuous, coordinated loud calls. These vocalizations appear to play a role in territorial defense, a hypothesis derived from studies of only three of the 31 recognized Callicebus species. Here, we describe the acoustic properties and organization of the loud calls of black-fronted titi monkeys ( Callicebus nigrifrons). We compare the behavioral and ecological contexts associated with these vocalizations to investigate their role in within- and between-group communication, resource defense, and mate defense. Black-fronted titi monkeys create loud calls by combining a finite number of syllables to form more complex phrases, which are assembled to compose long sequences of loud calls. Bioacoustic features distinguish the loud calls used in different contexts, involving communication within- and between-groups. We found support for the hypothesis that vocalizations used for between-group communication are cooperative displays used by the mated pair and other group members to regulate access to important food resources, such as fruits. On the other hand, we only found weak support for the mate defense hypothesis. Am. J. Primatol. 76:788-800, 2014. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Birds use song to communicate with conspecifics, and song can influence both intra-sexual competition and inter-sexual mate choice. Some birds produce repertoires consisting of hundreds of songs while others produce a single song type. For species with a single-song repertoire, there are varying levels of inter-individual variation which can be the result of environmental, genetic, and physiological factors. Male Snow Buntings (Plectrophenax nivalis) typically produce a single song type, and while syllable sharing occurs between individuals, many researchers have anecdotally noted the individuality of each male's song. To investigate this long-held assertion, we performed a detailed bioacoustic analysis of male Snow Buntings recorded in the Canadian Arctic. We use canonical discriminant analysis to provide quantitative evidence confirming that male Snow Buntings sing individually distinctive songs. Furthermore, we present the first evidence that some Snow Buntings exhibit inter-annual variation in song structure; while songs remain consistent within each year, two males changed their song type between years. Inter-individual variation in song content can have important behavioral implications, because it facilitates individual recognition and can affect individual fitness. [ABSTRACT FROM AUTHOR]

Noise pollution poses a significant obstacle to vocal communication. Songbirds rely on acoustic signals for mate choice and territory defense, and masking of these signals can have negative fitness consequences. Prior investigations reveal that birds mitigate the negative effects of acoustic masking by increasing their signal amplitude or by singing with higher minimum frequencies. In this study, we evaluate the responses of male Pacific Wrens (Troglodytes pacificus) to natural ambient noise (ocean surf) and anthropogenic noise (highway traffic) in Pacific Rim National Park Reserve, B.C., Canada. Pacific Wrens, known for their complex songs, are specialists of old-growth forest. We hypothesized that Pacific Wrens would compensate for the effects of ambient noise in their environments through modifications to their songs that enhanced their transmission properties. Recognizing that longer, higher frequency, and more complex signals propagate better in noisy environments, we predicted that Pacific Wrens would increase the length of their songs, the length of the syllables within their songs, the number of syllables per song, and the minimum frequency of their songs. Recordings of 52 territorial Pacific Wrens showed that proximity to highway traffic noise had a significant effect on song duration but no significant effect on any of the other measured variables. Pacific Wrens that were recorded near the shoreline, however, sang songs with longer syllables, and higher intra-individual variation in song duration. Number of syllables, syllable minimum frequency, and song duration did not vary with distance from the shoreline. We conclude that natural and anthropogenic noise sources influence the singing behavior of Pacific Wrens. [ABSTRACT FROM AUTHOR]

Phenotypic traits are important for assessing differences between populations, especially in groups with poorly resolved taxonomy. One such group, the House Wren complex, presents extensive taxonomic controversy and is thought to comprise many independent evolutionary units. Although the songs and morphological features of House Wrens (Troglodytes aedon) show extensive variation, differences between populations have not been quantified. We assessed variation in acoustic and morphometric traits within this complex and compared patterns of variation with currently recognized subspecies boundaries. First, we compared songs and morphology among eight recognized subspecies (T. a. aedon, T. a. parkmanii, T. a. cahooni, T. a. brunneicollis, T. a. nitidus, T. a. musculus, T. a. beam, and T. a. rufescens), controlling for significant effects of latitude. Second, we used variation in male song, a trait with an important role in mate choice and male-male competition, to assess divergence among subspecies. We compared variation among subspecies to variation across seven currently recognized Troglodytes species (T. hiemalis, T. pacificas, T. tanner!, T. sissonii, T. cobbi, T. rufociliatus, and T. ochraceus). Our results, based on broad sampling of songs (n = 786) and morphological traits (n = 401) from 609 locations throughout the Americas, show that most of the subspecies examined diverge in song, morphology, or both. In addition, the acoustic differences between subspecies are similar to, and in some instances greater than, the divergence between pairs of currently recognized species. Our results suggest that at least four allopatric subspecies-T. a. nitidus, T. a. musculus, T. a. beani, and T. a. rufescens-are likely different species, and we identify many other vocally and morphologically differentiated subspecies that may, upon further detailed genetic analysis, result in new species. [ABSTRACT FROM AUTHOR]

In multiple animal taxa, including many birds and primates, members of mated pairs produce coordinated acoustic displays known as duets. By observing the behaviour of territorial animals as they respond to playback-simulated duets of rivals, we can gain insight into the behavioural significance of vocal duets. Playback experiments, however, have been conducted across a very narrow range of duetting animals. Furthermore, many studies have been conducted with single-speaker playback, whereas stereo-speaker playback offers more spatially realistic simulation of duets. Moreover, by evaluating the reactions of animals to separate loudspeakers broadcasting male and female duet contributions, we can study the interactions of both males and females with same-sex vs. opposite-sex rivals. We used a paired experimental design to broadcast duet stimuli through a single-speaker and a stereo-speaker apparatus to 30 pairs of duetting barred antshrikes Thamnophilus doliatus in Costa Rica. Our goals were (1) to evaluate whether territorial antbirds respond more aggressively to male vs. female duet components and (2) to assess aggressive responses of antbirds towards single-speaker vs. stereo-speaker playback. Neither males nor females differentiated between the loudspeaker simulating the male vs. female duet contribution during stereo-speaker playback trials. Barred antshrikes displayed significantly stronger responses to stereo-speaker playback compared with single-speaker playback. Males displayed stronger playback responses than females with closer, quicker and more vocal responses. These results provide evidence for a joint resource defence function of antbird duets given that pairs responded together with equivalent intensity to male and female simulated intruders. This is the first study to show that although duetting is an aggressive territorial signal, birds do not necessarily respond to sex-specific components of duets. Our results support the idea that spatially realistic stereo presentation of duet stimuli is critical for experimental duet research. [ABSTRACT FROM AUTHOR]

Speciation may be influenced by geographic variation in animal signals, particularly when those signals are important in reproductive decisions. Here, we describe patterns of geographic variation in the song of rufous-naped wrens C ampylorhynchus rufinucha. This species complex is a morphologically variable taxon confined to tropical dry forest areas from Mexico to northwestern Costa Rica. Morphological and genetic analyses suggest that there are at least three partially isolated groups within the complex, including a secondary-contact zone in coastal western Chiapas between the subspecies C. r. humilus and C. r. nigricaudatus. Based on recordings throughout their geographic range, we investigate the effects of historical isolation on song structure and analyze whether genetic differences or climatic conditions explain observed patterns of variation. Our findings, based on a culturally-transmitted and sexually-selected trait, support the hypothesis that three evolutionary units exist within this taxon. Our results suggest that song differences between genetic groups were influenced by historical isolation. We report a strong relationship between vocal dissimilarity and genetic distance, suggesting that differences in vocal characteristics are probably affected by the same factors that drive genetic divergence. We argue that the evolution of song in this taxon is influenced by vicariant events, followed by accumulation of changes in song structure due to several possible factors: cultural drift in song structure; genetic drift in features related to song production; or natural selection acting on features that influence songs, such as body and beak size. [ABSTRACT FROM AUTHOR]

The article discusses the results of a study that examines the inconsistent geographic variation in Barred Owls or Strix varia's male calls, female calls and duets across the southeastern U.S. The molecular research showed two genetically distinct Barred Owl clades and calls were predicted to vary with genetic distance in a clinal pattern. However, duets and duetting behavior may show dialects similar to that of learned bird song.

Animals eavesdrop on dyadic interactions between other individuals to gather information for future mate choice and territory defense decisions. The capacity for eavesdroppers to combine information gathered from overhearing multiple two-way interactions is poorly studied. We tested whether inexperienced (second year) and older (after second year) male black-capped chickadees (Poecile atricapillus) eavesdrop on rivals' song contests to evaluate the relative threat levels of multiple unfamiliar territorial intruders. We used a multiple speaker playback experiment to simulate 3 male territorial intruders (A, B, and C) engaging in 2 successive dyadic song contests, presenting focal males with the information that A was more threatening than B, and B was more threatening than C. We then assayed the response of focal males when presented with simulated intruders A and C without relative information. We predicted that males would defend against the intruder perceived to be the greater threat. Focal males initially responded toward the more threatening intruder (A) significantly more than the less threatening intruder (C), consistent with our predictions. Older birds approached the more threatening intruder (A) significantly more than the less threatening intruder (C), whereas young males showed more variable responses. Our results suggest that male chickadees were able to acquire relative threat information from separate song contests that influenced their responses toward rivals paired in novel contests. These findings indicate that territorial songbirds in communication networks may be capable of integrating information gathered through eavesdropping on multiple interactions. [ABSTRACT FROM PUBLISHER]

Animals can encode information into signals using at least 2 basic mechanisms. First, signalers can repeat their signals, encoding information into sequence-level parameters, such as signaling rate. Second, signalers can encode information into the fine structural variation of individual signals. This mechanism requires sophisticated encoding and decoding but potentially affords more rapid or efficient information transfer. The chick-a-dee call of Parid birds is a structurally complex signal that conveys food- and predator-related information to both conspecific and heterospecific receivers. However, the basic mechanism by which it communicates information is unclear. Previous research suggests that variation in the number of terminal notes is important, but this structural trait has not been manipulated independently from other structural traits or from sequence-level parameters, such as total duty cycle. We independently manipulated the fine structure and duty cycle of the calls of Black-capped Chickadees (Poecile atricapillus) and then broadcast them to potential receivers. Both conspecific and heterospecific receivers ignored manipulations to the fine structure of individual calls when the duty cycle of the signaling sequences was held constant. In marked contrast, receivers exhibited significantly stronger responses when the duty cycle was experimentally increased, and the fine structure of individual calls was held constant. Specifically, signaling sequences with a high duty cycle attracted more conspecific and heterospecific receivers and caused those receivers to approach the speaker more quickly, to approach the speaker more closely, and to remain within 10 m of the speaker for longer. These findings show that receivers respond to a simple sequence-level trait in a structurally complex avian signal. [ABSTRACT FROM PUBLISHER]

Animals produce sounds for diverse biological functions such as defending territories, attracting mates, deterring predators, navigation, finding food and maintaining contact with members of their social group. Biologists can take advantage of these acoustic behaviours to gain valuable insights into the spatial and temporal scales over which individuals and populations interact. Advances in bioacoustic technology, including the development of autonomous cabled and wireless recording arrays, permit data collection at multiple locations over time. These systems are transforming the way we study individuals and populations of animals and are leading to significant advances in our understandings of the complex interactions between animals and their habitats. Here, we review questions that can be addressed using bioacoustic approaches, by providing a primer on technologies and approaches used to study animals at multiple organizational levels by ecologists, behaviourists and conservation biologists. Spatially dispersed groups of microphones (arrays) enable users to study signal directionality on a small scale or to locate animals and track their movements on a larger scale. Advances in algorithm development can allow users to discriminate among species, sexes, age groups and individuals. With such technology, users can remotely and non-invasively survey populations, describe the soundscape, quantify anthropogenic noise, study species interactions, gain new insights into the social dynamics of sound-producing animals and track the effects of factors such as climate change and habitat fragmentation on phenology and biodiversity. There remain many challenges in the use of acoustic monitoring, including the difficulties in performing signal recognition across taxa. The bioacoustics community should focus on developing a common framework for signal recognition that allows for various species' data to be analysed by any recognition system supporting a set of common standards. Synthesis and applications. Microphone arrays are increasingly used to remotely monitor acoustically active animals. We provide examples from a variety of taxa where acoustic arrays have been used for ecological, behavioural and conservation studies. We discuss the technologies used, the methodologies for automating signal recognition and some of the remaining challenges. We also make recommendations for using this technology to aid in wildlife management. [ABSTRACT FROM AUTHOR]

The article discusses a study in the vocal behavior and vocalizations of Barred Owls. It says that adult Barred Owls can make 13 distinct vocalizations and can also vocalize throughout daytime. It also mentions that most calls occurred at night in which type of vocalizations developed within duets and calls of females has longer terminal notes and higher vibrato than males. Moreover, cluster analysis specified two groups of owls through their sound differences into 91% accuracy predicted sex.

The ability to discriminate among signallers and to respond to them on an individual basis provides receivers with substantial benefits. For example, discriminating among signallers allows receivers to ignore unreliable individuals or to focus their territorial defence on unfamiliar intruders. Such discrimination requires signals to be individually distinctive; that is, signals must vary more among than within individuals. Furthermore, receivers must be able to discriminate among the signals of different individuals. In this study, we used fine structural analysis to show that the simple songs of male black-capped chickadees are individually distinctive, but that substantial variation exists both within and among recordings of the same individual. This finding emphasizes the need for multiple recordings of each individual in studies of individual distinctiveness, since failing to measure variation across recordings of the same individual can make it difficult to determine whether signals vary among individuals or whether they simply vary among different recording sessions. To test whether chickadees discriminate among the signals of different individuals, we used a playback experiment in which we broadcast priming and discrimination stimuli to 45 territorial males. When individuals heard the playback of two different males, they produced more songs and remained near the loudspeaker for a longer period than when they heard two different exemplars from the same male. Chickadees can therefore discriminate among singers based exclusively on their songs, which may help to explain how chickadees eavesdrop on singing contests and subsequently select extrapair mates on the basis of song contest performance. [Copyright &y& Elsevier]

Mated pairs of animals in many taxa coordinate their vocalizations into duets, yet most research on duetting has focused on songbirds. Here we examine the duetting behaviour of barred owls (Strix varia) by addressing three questions: (1) Do owl duets play a role in territorial interactions? (2) Do owls discriminate between duets of neighbours versus strangers? (3) Do duets play a role in extended communication among a neighbourhood of owls? We simulated territorial encounters by broadcasting duets of adjacent, territory-holding owls (neighbours) and distant owls (strangers). We assessed responses to playback using a 3.5-km transect of automated recording devices. We compared vocal activity during a pre-playback period and following both playback treatments for the focal pair, their neighbours, and more distant owls within the neighbourhood. After playback, focal owls gave significantly more duets, vocalized for a longer duration, and emphasized different call types compared to the pre-playback period, demonstrating that barred owls use duets in territory defence. Focal owls did not respond significantly differently to neighbours versus strangers. At the neighbourhood level, owls did not behave differently during silent pre-playback periods or post-playback periods. Our results suggest barred owl duets function primarily in immediate confrontations during territorial conflicts. [ABSTRACT FROM AUTHOR]

Animal signals are distorted as they transmit through the environment, but many species have evolved strategies to minimize distortion of vocal signals. Song structure can change over evolutionary time, or signallers can modify their behaviour, to reduce attenuation and degradation in a specific habitat. We investigated signalling behaviour in rufous-and-white wrens, Thyothorus rufalbus, a Neotropical songbird in which both sexes sing, with a specific focus on perch height. Based on previous findings, including a detailed sound transmission study, we predicted that rufous-and-white wrens would choose elevated song perches in order to maximize the distance their songs travel, and that males and females would show different perch height choices. We observed 30 birds in 18 territories in a tropical forest in Costa Rica to determine the perch heights of birds when singing, producing non-song vocalizations, collecting nesting material, foraging, and engaging in other behaviours. We found that both activity and sex influenced perch height. Birds sang from a variety of heights, but the highest perches were used as song posts by males. Males minimized singing from within 1 m of the ground, and they generally perched higher than females, which may reflect sex differences in communication strategies. Both sexes foraged on or near the ground. The finding that rufous-and-white wrens vary their perch height with specific activities suggests that they modify their singing behaviour to enhance long-distance communication. [ABSTRACT FROM AUTHOR]

Coordinated vocal displays of cooperatively breeding animals provide a compelling model for investigating the opposing motivations for engaging in conflict versus cooperative behaviours. Hypotheses for the function of coordinated vocal displays differ with respect to these motivations and have been traditionally investigated by using playback to simulate varying degrees of threat to individuals and groups. We evaluated the function of coordinated vocal displays by presenting territorial groups of cooperatively breeding rufous-naped wrens, Campylorhynchus rufinucha, with three playback stimuli: solos, duets and choruses. We found that all groups responded strongly to playback by approaching the loudspeaker together, vocalizing, and performing visual displays. A composite playback response measure showed significantly more aggressive reactions to all playback treatments compared to a preplayback control period, yet did not vary across solo, duet and chorus treatments. This suggests that the playback stimuli represented equally strong threats despite the varying numbers of contributors to each stimulus, and does not support the hypothesis that coordinated vocalizations are graded signals of threat in this species. Our findings stand in contrast to previous playback studies that have reported an increase in aggression with an increasing number of simulated intruders, or an increase in coordinated vocalizations in response to solo playback. We interpret the results of our study as evidence that coordinated vocalizations function in the cooperative behaviour of joint territory defence in the rufous-naped wren. [Copyright &y& Elsevier]

Summary: In many tropical animals, male and female breeding partners combine their songs to produce vocal duets . Duets are often so highly coordinated that human listeners mistake them for the songs of a single animal . Behavioral ecologists rank duets among the most complex vocal performances in the animal kingdom . Despite much research, the evolutionary significance of duets remains elusive , in part because many duetting animals live in tropical habitats where dense vegetation makes behavioral observation difficult or impossible. Here, we evaluate the duetting behavior of rufous-and-white wrens (Thryothorus rufalbus) in the humid forests of Costa Rica. We employ two innovative technical approaches to study duetting behavior: an eight-microphone acoustic location system capable of triangulating animals'' positions on the basis of recordings of their vocalizations and dual-speaker playback capable of simulating duets in a spatially realistic manner . Our analyses provide the first detailed spatial information on duetting in both a natural context and during confrontations with rivals. We demonstrate that birds perform duets across highly variable distances, that birds approach their partner after performing duets, and that duets of rivals induce aggressive, sex-specific responses. We conclude that duets serve distinct functions in aggressive and nonaggressive contexts. [Copyright &y& Elsevier]

We provide the first detailed analysis of the vocal behavior of Pacific coast Plain Wrens (Thryothorus modestus modestus). Male Plain Wrens sing two categories of song phrase; one phrase is given as a solo song or duet introduction (I-phrases), while the other is sung primarily during duets (M-phrases). Females sing only one category of song phrase, usually during duets (F-phrases). Both sexes have repertoires of their respective song categories. Plain Wren duets show extreme temporal coordination: duets typically begin with male I-phrases, which are followed by the rapid alternation of female F-phrases and male M-phrases. These patterns are congruent with the solo and duet singing behavior of a sister taxon to the Plain Wren, the Canebrake Wren (Thryothorus modestus zeledoni). Our analyses of Plain Wren songs and duets reveal pronounced differences between males and females and contribute to our understanding of the complex duetting behavior of Thryothorus wrens. [ABSTRACT FROM AUTHOR]

Dominance relationships structure many animal societies, yet the process of rank attainment is poorly understood. We investigated acquisition of social dominance in winter flocks and its fitness consequences in male black-capped chickadees (Poecile atricapillus) over a 10-year period. Age was the best predictor of rank, and paired comparisons showed high-ranked males to be older than their low-ranked flock-mates. When controlling for age, morphological variables did not predict male social rank, but high-ranked males were heavier, had lower fat scores and were in leaner condition than low-ranked males. Males that survived between years tended to increase in rank over time; however, the rate of rank advancement varied individually. Rank reversals between familiar contestants were rare, and changes in male social rank were associated with changes in flock membership. Average lifetime reproductive success (LRS) of males and females was variable and best predicted by lifespan. Male rank history also influenced realized reproductive success. Birds with higher average rank over their lifespan were more likely to reproduce successfully. However, among successful birds, average rank did not significantly predict LRS. Thus, birds that lived longer and attained high social rank earlier had higher fitness, but this effect was not manifested as fine-scale differences among successful individuals. Taken together, these findings demonstrate the importance of social factors influencing individual fitness. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 85–95. [ABSTRACT FROM AUTHOR]

Animal signals are characterized by two design components: efficacy (detectability) and content (message being conveyed). Selection for efficient signal perception should favor the evolution of traits that exhibit an optimal balance between these two design components. We examined the evolution of signal design in the colorful plumage ornaments of manakins (Aves: Pipridae). We used a model of avian color space to quantify how differences in plumage coloration would be perceived by a typical passerine bird and examined patterns of coloration across 50 species of manakin. Using phylogenetically independent contrasts, we show that plumage contrast against the background increases with sexual dichromatism in males but not females, suggesting that sexual selection has favored the evolution of male plumage ornaments that enhance signal efficacy. Plumage contrast within individuals also increased with dichromatism in males but not females. Finally, plumage colors produced by different mechanisms, which may reveal different aspects of quality, resulted in different degrees of contrast against the background. Our findings suggest that selection for signal efficacy and content may sometimes be opposing, creating a trade-off between these two components of signal design. Manakins may mediate this trade-off by combining multiple plumage ornaments that differ in efficacy and content. [ABSTRACT FROM AUTHOR]

A field test was conducted on the accuracy of an eight-microphone acoustic location system designed to triangulate the position of duetting rufous-and-white wrens (Thryothorus rufalbus) in Costa Rica’s humid evergreen forest. Eight microphones were set up in the breeding territories of 20 pairs of wrens, with an average intermicrophone distance of 75.2±2.6 m. The array of microphones was used to record antiphonal duets broadcast through stereo loudspeakers. The positions of the loudspeakers were then estimated by evaluating the delay with which the eight microphones recorded the broadcast sounds. Position estimates were compared to coordinates surveyed with a global-positioning system (GPS). The acoustic location system estimated the position of loudspeakers with an error of 2.82±0.26 m and calculated the distance between the “male” and “female” loudspeakers with an error of 2.12±0.42 m. Given the large range of distances between duetting birds, this relatively low level of error demonstrates that the acoustic location system is a useful tool for studying avian duets. Location error was influenced partly by the difficulties inherent in collecting high accuracy GPS coordinates of microphone positions underneath a lush tropical canopy and partly by the complicating influence of irregular topography and thick vegetation on sound transmission. [ABSTRACT FROM AUTHOR]

The elaborate songs of male animals are thought to function in either territory defense (male–male communication) or mate attraction (male–female communication). In non-territorial animals, male vocalizations are expected to function primarily in mate attraction, yet the reproductive consequences of male vocalizations in non-territorial animals are poorly described. Here we explore the relationship between male song and male reproductive performance in a free-living population of house finches, Carpodacus mexicanus, a non-migratory, non-territorial songbird. Based on recordings of 20 males, we analyzed three song features (song length, number of unique syllables per song, and song rate) and compared male song with two measures of within-pair reproductive performance (nest initiation date and clutch size) and one measure of extra-pair reproductive performance (whether males sired extra-pair young). We demonstrate a positive association between male song and within-pair reproductive performance; males that sang long songs initiated their first clutch significantly earlier and males that sang songs at a faster rate had larger clutches. Despite the fact that only one of our recorded males sired extra-pair young in the nest of another male, this male's songs were the most elaborate for two of three song features measured, anecdotally suggesting that male song may play a role in both within-pair and extra-pair partner choice. These results suggest that male song is a sexually selected trait in non-territorial house finches. [ABSTRACT FROM AUTHOR]

Geographic variation in male bird songs has been studied extensively, but there have been few investigations of geographic variation in female songs or sex differences in patterns of geographic variation. We compared patterns of variation in male and female songs of eastern whipbirds Psophodes olivaceus by analyzing recordings from 16 populations across the species’ geographic range in eastern Australia. We found remarkably different patterns of geographic variation between the sexes. Female eastern whipbird songs are easily categorized into discrete song types. Song types are shared between nearby females, but female songs show pronounced differences at a continental scale. In contrast, male eastern whipbird songs show high consistency throughout the species’ geographic range. All recorded males share the ability to transpose the frequency of the introductory whistle and most recorded males share the ability to vary the direction of the slope of the terminal whip crack. For eight of nine measured variables, female songs show significantly higher levels of variation than male songs. We discuss whether sex differences in dispersal, song learning strategies, and song function may explain these sex differences in patterns of song variation. We suggest that eastern whipbirds have experienced a decoupling of male and female song learning strategies and that the songs of each sex have responded to different selective pressures in the context of countersinging interactions. We speculate that consistency in male songs throughout the geographic range of eastern whipbirds may arise through female preference for males that perform large bandwidth whip cracks. [ABSTRACT FROM AUTHOR]

Hypotheses for the function of animal vocal duets fall into three broad categories. Male and female breeding partners may use duets to communicate with each other, to communicate with same-sex outsiders, or to communicate with opposite-sex outsiders. To evaluate these categories of duet function, I gave stereo duet playback to territorial pairs of duetting rufous-and-white wrens, Thryothorus rufalbus, in northwestern Costa Rica. I simulated duets of a rival pair of wrens by simultaneously broadcasting male and female duet components through separate loudspeakers. Territorial males and females responded aggressively to duet playback by rapidly approaching the speakers and singing both solo songs and duets. Males sang more songs in response to playback than females, but both sexes responded to approximately half of their partner''s songs to create duets. The aggressive responses of territorial pairs were consistent with a territorial defence hypothesis for duet function. Males spent more time near the male speaker and approached the male speaker more closely, suggesting that male duet contributions play a role in intrasexually aggressive extrapair communication. Females approached the male and female speakers with similar intensity, although they tended to respond more strongly to the male speaker. In the few cases where females responded independently of their partner, they responded more strongly on the side of the female speaker. Taken together, responses of rufous-and-white wrens to stereo duet playback suggest that duets play a role in territory defence against conspecific rivals, and, for males, duets may play an additional role in mate guarding and paternity guarding. [Copyright &y& Elsevier]

In many tropical bird species, males and females sing together in coordinated vocal duets. Although studies of duetting present unique opportunities for understanding conflict and cooperation between the sexes, very few investigations describe the similarities and differences between male and female singing behaviors. Here, we present the first detailed account of the singing behavior of Rufous-and-white Wrens (Thryothorus rifalbus), a resident tropical duetting songbird. Male and female songs share a similar structure, yet show pronounced sex differences. Male songs have lower frequency characteristics and more repeated trill syllables, and often sound louder than female songs. Males sing more than females, and only males show elevated song output at dawn. Both males and females have song repertoires. Males have an average repertoire size of 10.8 song types, whereas females have a significantly smaller average repertoire size of 8.5 song types. Although males share proportionately more of their song types with neighbors than females do, both sexes share more song types with nearby individuals than with distant individuals. Breeding partners combine their solo songs to create duets. Duets assume a variety of different forms, ranging from simple, overlapping male and female songs to complex combinations of multiple male and female songs. Most duets (73%) are created by females responding to male song. Males respond to female-initiated duets with shorter latencies than when females respond to male-initiated duets. Each pair sings certain combinations of song types in duets more often than can be explained by random association, which demonstrates that Rufous-and-white Wrens have duet types. The most common duet type was different for each pair Our results show that Rufous-and-white Wrens have pronounced sex differences in song structure, singing activity, repertoire size, repertoire sharing, and duetting behavior. [ABSTRACT FROM AUTHOR]

Birds that nest in cavities may regulate nest microclimate by orienting their nest entrance relative to the sun or prevailing winds. Alternatively, birds may orient their nest entrance relative to conspecific individuals around them, especially if the acoustic properties of cavities permit nesting birds to better hear individuals in front of their nest. We measured the cavity entrance orientation of 132 nests and 234 excavations in a colour-banded population of black-capped chickadeesPoecile atricapillusfor which the reproductive behaviour of nesting females was known. Most chickadees excavated cavities in rotten birchBetula papyrifera, aspenPopulus tremuloidesand mapleAcer saccharum. Nest cavities showed random compass orientation around 360° demonstrating that chickadees do not orient their cavities relative to the sun or prevailing winds. We also presented chickadees with nest boxes arranged in groups of four, oriented at 90° intervals around the same tree. Nests constructed in these nest box quartets also showed random compass orientation. To test the acoustic properties of nest cavities, we conducted a sound transmission experiment using a microphone mounted inside a chickadee nest. Re-recorded songs demonstrate that chickadee nest cavities have directional acoustic properties; songs recorded with the cavity entrance oriented towards the loudspeaker were louder than songs recorded with the cavity entrance oriented away from the loudspeaker. Thus, female chickadees, who roost inside their nest cavity in the early morning during their fertile period, should be better able to hear males singing the dawn chorus in front of their nest cavity. Using GIS analyses we tested for angular-angular correlation between actual nest cavity orientation and the azimuth from the nest tree to the territories and nest cavities of nearby males. In general, nest cavity entrances showed no angular-angular correlation with neighbourhood territory features. However, among birds who followed a mixed reproductive strategy and nested in the soft wood of birch and aspen trees, nest cavity entrances were oriented towards their extra-pair partners. We conclude that nest cavity orientation in birds may be influenced by both ecological and social factors. [ABSTRACT FROM AUTHOR]

During vocal contests, animals alter both the timing and the patterning of responses to their opponents. Time-specific responses (such as overlapping an opponent''s song) and pattern-specific responses (such as matching the type of an opponent''s song) may reveal information about interacting animals. Here we explore the consequences of overlapping and frequency matching during song contests of male black-capped chickadees, Poecile atricapillus. Using interactive playback, we engaged birds of high and low dominance status in vocal interactions with a simulated territorial intruder. The playback intruder either overlapped or avoided overlapping the subjects'' songs and either matched or avoided matching the frequency of the subjects'' songs. Individuals who were overlapped by the playback intruder showed higher variability in their song length and song timing than individuals who were not overlapped. Individuals who were frequency-matched by the playback intruder responded with more agitated responses (more flights and passes over the speaker and closer distances of closest approach) and spent more time farther away from the loudspeaker. We argue that the timing of song delivery and the choice of song type are distinct functional components of vocal interactions, where overlapping and matching songs are threatening signals that have separate consequences for opponent behaviour and song performance. High-ranking males responded at greater distances from the loudspeaker in all treatments and responded with lower agitation levels than low-ranking males. We demonstrate that males of different quality show different behavioural responses to territorial intruders, where males of high status appear reluctant to engage an intruder as intensely as males of low status. [Copyright &y& Elsevier]

The two-note fee-bee song of male black-capped chickadees functions during the dawn chorus, in part, as a sexual signal across large distances. How song structure might encode information about male quality, however, remains unclear. We studied the availability of cues to male social rank (a proxy indicator of male quality), within the acoustic structure of dawn chorus songs of male chickadees whose flock dominance status we determined the previous winter. We used analysis of variance and discriminant function analysis to demonstrate that five temporal, frequency or relative amplitude features of song can predict individual identity but not the category of social rank (dominant versus subordinate) to which individuals belong. After transmitting chickadee songs through the forest and re-recording them at four broadcast distances, we found that song structure continued to effectively predict singer identity by our statistical methods despite significant acoustic degradation for as long as songs remained audible (up to 80 m). In particular, the relative frequency interval between the two notes is both the most invariant between-male measure and among the most individually distinctive. We conclude the structure of dawn chorus songs could function across large distances to signal the identity of familiar singing males whose relative quality is known to the listener from other interactions (such as encounters within winter flocks). [ABSTRACT FROM AUTHOR]

Within a network of communicating individuals, animals may gather information about the relative quality of conspecifics by eavesdropping on their signalling interactions. For territorial male songbirds, eavesdropping may be a low-cost, low-risk method for assessing the relative quality of the males around them. We used a three-speaker playback design to evaluate whether male black-capped chickadees (Poecile atricapillus) respond differently to two simulated countersinging intruders who differ only in relative features of their singing performance. We arranged three loudspeakers in an equilateral triangle at the center of playback subjects' territories. After luring males to the first loudspeaker by broadcasting non-song vocalizations, we played songs from the remaining loudspeakers to simulate a countersinging interaction between two male intruders. During the interactions, one simulated intruder consistently overlapped the songs of the other, a behaviour thought to be a signal of directed aggression in songbirds. Territorial male chickadees discriminated between the simulated intruders by preferentially approaching the loudspeaker broadcasting the overlapping signal, suggesting that males eavesdrop on other males' countersinging interactions. Male responses to playback support the idea that overlapping is a more threatening signal than being overlapped. Responses varied with the dominance status of the subject. High-ranking males approached the overlapping loudspeaker in 15 of 16 cases whereas low-ranking males approached the overlapping speaker in only 5 of 10 cases, suggesting that males of different quality may use different tactics for territorial defense. [ABSTRACT FROM AUTHOR]

Optimal sex allocation for individuals can be predicted from a number of different hypotheses. Fisherian models of sex allocation predict equal investment in males and females up to the end of parental care and predict brood compositions based on the relative costs of producing males and females. The Trivers-Willard hypothesis predicts that individual females should alter the sex ratio of their broods based on their own condition if it has a differential impact on the lifetime reproductive success of their sons and daughters. The Charnov model of sex allocation predicts that females should alter sex allocation based on paternal attributes that may differentially benefit sons versus daughters. Because females are the heterogametic sex in birds, many recent studies have focussed on primary sex ratio biases. In black-capped chickadees Poecile atricapilla , males are larger than females suggesting they may be more costly to raise than females. Female condition affects competitive ability in contests for mates, and thus may be related to variance in fecundity. Females prefer high-ranking males as both social and extrapair partners. These observations suggest that females might vary the sex ratio of their broods based on the predictions of any of the above models. Here, we report on the results of PCR based sex determination of 1093 nestlings in 175 broods sampled from 1992 to 2001. Population-wide, we found a mean brood sex ratio of 0.525±0.016, with no significant deviation from a predicted binomial distribution. We found no effect of clutch size, female condition, hatch date, parental rank or paternity. Our results reject the idea that female black-capped chickadees systematically vary sex allocation in their broods. [ABSTRACT FROM AUTHOR]

Research on anuran parasites often focuses on the description and identification of parasites and demonstrating that parasitic infection can have deleterious effects on the health of amphibians. There is comparatively little research on how parasitism influences evolutionary dynamics such as sexual selection. Here, we explore anuran blood parasites with a focus on parasite-mediated sexual selection. The Hamilton–Zuk hypothesis posits that parasite–host coevolution drives sexual ornamentation of hosts, and that the degree of ornament expression honestly signals heritable parasite resistance. Although the Hamilton–Zuk hypothesis has received support in a number of animal taxa, it has received little attention in amphibians. Neotropical Yellow Toads (Incilius luetkenii) are sexually dichromatic toads with an unusual sexual ornament: males turn from cryptic brown to bright lemon yellow for a very short period during the brief breeding season. In this study, we identify blood parasites in male Neotropical Yellow Toads, and we investigate one component of the Hamilton–Zuk hypothesis by comparing skin color and body condition to blood parasite levels. We found four types of parasites. Apicomplexa and frog erythrocytic virus were the most common, infecting 73% and 46% of toads, respectively, whereas Nematoda and bacteria were found in 27% and 36% of toads, respectively. We found negative relationships between parasites and male color: males infected with fewer Apicomplexa and Nematoda had higher yellow saturation, and males with fewer Nematoda had longer-wavelength hue. In contrast, we found one positive relationship between parasites and male color: males infected with higher levels of frog erythrocytic virus had higher brightness. Our results suggest that parasites may influence sexual traits in this species, but that the relationship is complex. This research contributes to our understanding of amphibian host–parasite coevolution by demonstrating a relationship between parasites and host coloration. [ABSTRACT FROM AUTHOR]

• We explore mechanisms and function of dynamic colour change in tropical yellow toads. • In an arena experiment, we test the influence of conspecific animals and hormones. • Toads held with conspecifics were significantly brighter than toads held in isolation. • Across treatments, the darkest toads had significantly higher levels of plasma corticosterone. • Toads became darker across subsequent days at the start of the breeding season. Dynamic sexual dichromatism occurs when males and females differ in colouration for a limited time. Although this trait has been primarily studied in cephalopods, chameleons, and fishes, recent analyses suggest that dynamic dichromatism is prevalent among anurans and may be mediated through sexual selection and sex recognition. Yellow toads, Incilius luetkenii , exhibit dynamic dichromatism during explosive breeding events at the onset of the rainy season: males change from a cryptic brown to a bright yellow and back again during the brief mating event. We tested the hypothesis that dynamic dichromatism in yellow toads is influenced by conspecific interactions and mediated through sex hormones and stress hormones. We placed male toads into one of four social treatments (with three other males, one male, one female, or no other toads). Immediately before and after each one-hour treatment, we quantified male colour with a reflectance spectrometer and we collected a blood sample to assess plasma concentrations of both testosterone and corticosterone. We found that males held with conspecific animals showed the brightest yellow colour and showed little or no change in their corticosterone levels. Across treatments, toads with duller yellow colour had higher levels of corticosterone. Male colour showed no association with testosterone. Interestingly, males showed substantial temporal variation in colour and corticosterone: toads were duller yellow and exhibited greater levels of corticosterone post-treatment across subsequent days at the onset of the rainy season. Our findings reveal that both conspecific interactions and corticosterone are involved in the dynamic colour change of yellow toads. [ABSTRACT FROM AUTHOR]

Animals' learned vocalizations often show high levels of intraspecific variation, and different variants may play different roles in attracting mates and settling territorial disputes. In any animal population with variable vocalizations, certain acoustic variants may be common and others rare. The responses of animals to local versus nonlocal sounds have been well characterized in prior investigations, but few investigations have explored the reactions of animals to songs that are present at different frequencies of occurrence within a population. Using an experimental approach to investigate a breeding population of Savannah sparrows, Passerculus sandwichensis , we tested the responses of territorial male songbirds to playback of locally common songs, locally rare songs and heterospecific songs. Male sparrows showed strong responses to conspecific song but weak responses to heterospecific song. Males showed their strongest responses to locally common songs and weaker responses to locally rare songs. Birds did not appear to discriminate between playback songs based on whether they were of the same song type as their own. We suggest that male Savannah sparrows perceive locally rare songs as weaker threats than locally common songs, similar to birds' reactions to foreign dialects. This could be due to low familiarity with locally rare songs, reduced female preference for locally rare songs, decreased success with territory defence for males singing locally rare songs or other perceived differences between locally rare and locally common songs. Overall, our results suggest that different song types elicit different responses from territorial songbirds, even songs that are found within the same population with different frequencies of occurrence. • Many animals show small-scale geographical variation in vocalizations. • Responses to locally common or locally rare vocalizations are seldom explored. • We used playback to test birds' responses to locally common and locally rare songs. • Savannah sparrows responded more strongly to locally common songs. • We suggest locally common songs are perceived as a stronger threat. [ABSTRACT FROM AUTHOR]

Male songbirds of many species sing local song dialects that are restricted to defined geographical areas. In most tests of responses to local versus foreign dialects, males respond more aggressively to songs from their own dialect, presumably because local males represent more of a threat to their success. We asked how hearing foreign songs during development and territory establishment affects discrimination of the local dialect in wild Savannah sparrows, Passerculus sandwichensis. After foreign songs had been heard from loudspeakers in the study area in at least two consecutive breeding seasons, males reduced the intensity of their responses to the local version of the population-specific buzz segment of the song. Four years after the foreign songs were last broadcast on the study area, males again responded more aggressively to the local version of the buzz. As for the basis of these responses, we found no evidence that birds discriminated among dialects by comparing them to their own songs. However, auditory experience with a foreign song, whether during song development (from speaker-simulated song tutors) or during the current breeding season (from neighbours' songs), reduced the intensity of birds' responses to the local buzz type. Both familiarity, in the form of auditory experience with a song type, and homogeneity, when a song type is sung by all or nearly all of the population, appear to contribute to heightened aggressive responses to a local song dialect. • We studied discrimination of local vs foreign dialects in wild Savannah sparrows. • Responses to the local dialect were weaker when the population's songs were diverse. • Responses to the local dialect increased with dialect homogeneity. • A male's own song type did not influence his responses to local and foreign songs. • Auditory experience during development and adulthood affected dialect discrimination. [ABSTRACT FROM AUTHOR]

Diverse animal species engage in long‐distance migrations. Many migrants travel in groups, and communication within these groups may be important to survival and successful migration. We examined migration and communication in 36 species of wood warblers (Parulidae), songbirds that breed in North America and migrate in mixed‐species flocks to their wintering grounds. During migration, wood warblers produce short vocalizations called "flight calls." The function of flight calls and the patterns of acoustic similarity between species are poorly understood. We investigated whether acoustic similarity of flight calls of different species of warbler reflects the similarity in their migratory journeys or their phylogenetic relatedness. We found that phylogeny, similarity in breeding latitude, and overlap in the timing of migration predict acoustic flight call similarity across warbler species. Further, we found that phylogeny, similarity in migration distance, and overlap in wintering range predict acoustic flight call similarity in a subset of 12 species with highly similar calls, although this analysis has a small sample size. We conclude that migratory similarity may be an important force driving the evolution of acoustically similar calls in wood warblers, in addition to phylogenetic relatedness. Acoustic convergence in these species may facilitate communication between individuals with similar migrations. [ABSTRACT FROM AUTHOR]

Summary In eight groups of animals, including humans and songbirds, juveniles are understood to learn vocalizations by listening to adults [ 1–4 ]. Experimental studies of laboratory-reared animals support this hypothesis for vocal learning [ 5–7 ], yet we lack experimental evidence of vocal learning in wild animals. We developed an innovative playback technology involving automated loudspeakers that broadcast songs with distinctive acoustic signatures. We used this technology to simulate vocal tutors in the wild and conducted year-long tutoring sessions to five cohorts of free-living migratory Savannah Sparrows in eastern Canada. We confirm that wild birds learn songs by listening to adult conspecific animals, and we show that they pass these songs on to subsequent generations. Further, we provide the first experimental evidence in the wild that the timing of exposure to tutor song influences vocal learning: wild Savannah Sparrows preferentially learn songs heard during both their natal summer and at the outset of their first breeding season. This research provides direct experimental evidence of song learning by wild animals and shows that wild birds learn songs during two critical stages of development early in life. Video Abstract Graphical Abstract Highlights • Wild Savannah Sparrows learn songs from playback-simulated vocal tutors • Birds preferentially learn songs heard in both their natal summer and first spring • Birds pass experimentally introduced songs on to subsequent generations Using playback to broadcast novel songs, Mennill et al. show that wild Savannah Sparrows learn songs by listening to adult tutors. Birds preferentially learn songs heard during both their natal summer and the outset of their first breeding season. Birds that learn from simulated vocal tutors also pass these songs on to subsequent generations. [ABSTRACT FROM AUTHOR]

Animal culture changes over time through processes that include drift, immigration, selection, and innovation. Cultural change has been particularly well-studied for animal vocalizations, especially for the vocalizations of male animals in the temperate zone. Here we examine the cultural change in the vocalizations of tropical Rufous-and-white Wrens (Thryophilus rufalbus), quantifying temporal variation in song structure, song type diversity, and population-level distribution of song types in both males and females. We use data from 10 microsatellite loci to quantify patterns of immigration and neutral genetic differentiation over time, to investigate whether cultural diversity changes with rates of immigration. Based on 11 yr of data, we show that the spectro-temporal features of several widely-used persistent song types maintain a relatively high level of consistency for both males and females, whereas the distribution and frequency of particular song types change over time for both sexes. Males and females exhibit comparable levels of cultural diversity (i.e. the diversity of song types across the population), although females exhibit greater rates of cultural change over time. We found that female changes in cultural diversity increased when immigration is high, whereas male cultural diversity did not change with immigration. Our study is the first long-term study to explore cultural evolution for both male and female birds and suggests that cultural patterns exhibit notable differences between the sexes. [ABSTRACT FROM AUTHOR]

When animals vocalize under the threat of predation, variation in the structure of calls can play a vital role in survival. The chick-a-dee calls of chickadees and titmice provide a model system for studying communication in such contexts. In previous studies, birds' responses to chick-a-dee calls covaried with call structure, but also with unmeasured and correlated parameters of the calling sequence, including duty cycle (the proportion of the calling sequence when a signal was present). In this study, we exposed flocks of Black-capped Chickadees (Poecile atricapillus) and heterospecific birds to playback of chick-a-dee calls and taxidermic models of predators. We quantified birds' responses to variation in number of D notes and duty cycle of the signalling sequence. Chickadees and heterospecific birds responded more intensely to high-duty-cycle treatments, and equally to treatments where duty cycle was held constant and the number of D notes varied. Although our study does not disentangle the effects of call rate and duty cycle, it is the first to investigate independently the behavioural responses of birds to variation in structural and sequence-level parameters of the chick-a-dee call during a predator confrontation. Critically, our results confirm that the pattern previously observed in a feeding context holds true in a mobbing context: variation in calling sequences, not in call structure, is the salient acoustic feature of chick-a-dee calls. These results call into question the idea that chick-a-dee call structure carries allometric information about predator size, suggesting instead that sequence-level parameters play a central role in communication in a mobbing context. [ABSTRACT FROM AUTHOR]