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Acoustic communication is fundamentally constrained by noise. In birds, the masking of mating signals (songs) may reduce fitness and, hence, they have evolved various mechanisms to maintain communication in noise. One of these tactics is the adjustment of song timing to avoid overlap with masking sounds, but previous studies yielded contrasting results regarding the occurrence and magnitude of this behavioural plasticity. Here, we investigated how temporal noise avoidance varies with noise intensity. We exposed singing Canaries (Serinus canaria) to playbacks of masking noise of varying amplitude [60–80 dB(A) SPL]. Contrary to our prediction, the birds did not shift song onsets to the silent intervals between noise bursts at high noise amplitudes, nor did they increase singing outside the playback period. Rather, we found that noise generally triggered the onset of song: the Canaries preferentially sang during the noise bursts independent of the noise amplitude. This behaviour is somewhat paradox because it leads to the most unfavourable signal-to-noise ratios. Our results, together with findings from the current literature, indicate marked species differences in the noise-induced song plasticity of birds. Therefore, we suggest a more comprehensive conception of noise that incorporates both supressing and stimulating effects.

Biomedical and social scientists are increasingly calling the biological sex into question, arguing that sex is a graded spectrum rather than a binary trait. Leading science journals have been adopting this relativist view, thereby opposing fundamental biological facts. While we fully endorse efforts to create a more inclusive environment for gender-diverse people, this does not require denying biological sex. On the contrary, the rejection of biological sex seems to be based on a lack of knowledge about evolution and it champions species chauvinism, inasmuch as it imposes human identity notions on millions of other species. We argue that the biological definition of the sexes remains central to recognising the diversity of life. Humans with their unique combination of biological sex and gender are different from non-human animals and plants in this respect. Denying the concept of biological sex, for whatever cause, ultimately erodes scientific progress and may open the flood gates to "alternative truths."

The impacts of noise pollution on birdsong have been extensively investigated but potential long-term effects are neglected. Near airports, where noise levels are particularly high, birds start singing earlier in the morning, probably to gain more time of uninterrupted singing before air traffic sets in. In a previous study, we documented this phenomenon in the vicinity of Berlin Tegel airport. In 2020, Tegel airport closed down, giving us the opportunity to investigate potential long-term effects after noise removal and to gain insight into the mechanisms underlying the advancement of dawn singing. We found that several species at the airport shifted their song onset back after the closure and now had similar schedules to their conspecifics at a control site. Some species, however, still sang earlier near the closed airport. While the first suggests plastic adaptation, the latter suggests selection for early singing males in areas with long-lasting noise pollution. Our findings indicate that a uniform behavioural response to anthropogenic change in a community can be based on diverging evolutionary mechanisms. Overall, we show that noise pollution can have long-lasting effects on animal behaviour and noise removal may not lead to immediate recovery in some species.

Many animals use acoustic signals for communication, implying that the properties of these signals can be under strong selection. The acoustic adaptation hypothesis predicts that species living in dense habitats emit lower-frequency sounds than those in open areas, because low-frequency sounds generally propagate further in denser vegetation. Signal frequency may also be under sexual selection, because it correlates with body size and lower-frequency sounds are perceived as more intimidating. Here, we evaluate these hypotheses by analysing variation in peak song frequency across 5,085 passerine species (Passeriformes). A phylogenetically-informed analysis revealed that song frequency decreases with increasing body mass and with male-biased sexual size dimorphism. However, we found no support for the predicted relationship between frequency and habitat. Our results suggest that the global variation in passerine song frequency is mostly driven by natural and sexual selection causing evolutionary shifts in body size rather than by habitat-related selection on sound propagation.Statement of authorshipTA and PM conceived and designed the study with input from all authors. TA and BK coordinated the study. PM collected the song data. MV performed the statistical analyses with input from WF. MB made the figures with help from MV and PM. TP and HB provided bioacoustic expertise. PM drafted the first version of the manuscript. TA, BK and PM revised and finalized the manuscript with input from all authors.Data availability statementThe data used in this study were collected from publicly available databases. All data and computer code used to generate the results, as well as supplementary figures and tables will be freely available at https://osf.io/fa9ky/.

The broken-wing display is a well-known and conspicuous deceptive signal used to protect birds' broods against diurnal terrestrial predators. Although commonly associated with shorebirds, it remains unknown how common the behaviour is across birds and what forces are associated with the evolution of the display. Here, we use the broken-wing display as a paradigmatic example to study the evolution of a behaviour across Aves. We show that the display is widespread: it has been described in 52 families spread throughout the phylogeny, suggesting that it independently evolved multiple times. Further, we evaluated the association with 16 ecological and life-history variables hypothesized to be related to the evolution of the broken-wing display. Eight variables were associated with the display. We found that species breeding farther from the equator, in more dense environments, with shorter incubation periods, and relatively little nest cover were more likely to perform the display, as were those in which only one parent incubates eggs, species that mob nest predators and species that are altricial or multi-brooded. Collectively, our comprehensive approach identified forces associated with the repeated evolution of this conspicuous display, thereby providing new insights into how deceptive behaviours evolve in the context of predator–prey interactions.

Traffic noise leads to vocal learning deficits in young songbirds, the primary model for speech development in humans., Noise pollution has been linked to learning and language deficits in children, but the causal mechanisms connecting noise to cognitive deficiencies remain unclear because experimental models are lacking. Here, we investigated the effects of noise on birdsong learning, the primary animal model for vocal learning and speech development in humans. We found that traffic noise exposure retarded vocal development and led to learning inaccuracies. In addition, noise suppressed immune function during the sensitive learning period, indicating that it is a potent stressor for birds, which is likely to compromise their cognitive functions. Our results provide important insights into the consequences of noise pollution and pave the way for future studies using birdsong as an experimental model for the investigation of noise-induced learning impairments.

Abstract It has often been observed that birds sing at a higher pitch in cities and other areas that are polluted with intense low-frequency noise. How this pattern arises remains unclear though. One prevailing idea is that songbirds adjust song frequencies to environmental noise profiles through developmental plasticity via vocal learning. However, the conclusions of previous studies testing this hypothesis are inconsistent. Here we report the findings from two song learning experiments with zebra finches (Taenopygia guttata), in which we exposed young birds to anthropogenic noise during their sensitive vocal learning period. Unlike previous studies that addressed this issue, we did not use constant synthetic noise but natural urban noise with its typical amplitude fluctuations that was broadcast at realistic sound levels. We found that noise-exposed males in neither experiment developed higher pitched songs compared to control males. This suggests that the natural fluctuations between higher and lower noise levels in cities may allow young birds to exploit relatively quiet moments to hear their tutors and themselves, permitting them to make accurate copies of even low-frequency song elements. Significance statement If animals are to persist in urban habitats, they often must adjust their behavior to the altered conditions. Birds in cities are often observed to sing at a higher pitch, but we are largely ignorant of how this phenomenon arises. We investigated whether low-frequency traffic noise interferes with the song learning of birds so that they develop higher pitched songs. Accordingly, we played back natural traffic noise from urban bird habitats to young birds during their learning period and then analyzed their adult songs. We found that birds that learned their songs in noise did not sing at higher frequencies compared to control males that learned their song with no noise exposure. Our results show that typical traffic noise in cities may not be sufficient to interfere with vocal learning in a way that birds develop higher-pitched songs.

Collective signalling occurs in diverse animal groups. A particularly well-studied form of this sophisticated communication behaviour is vocal duetting in birds, in which members of a mated pair coordinate their songs on short temporal scales. Many hypotheses have been proposed to explain the function of bird duets, but the experimental evidence for them is still somewhat conflicting. Studies on songbirds often provide support for the joint resource defence or mate-guarding hypotheses. We investigated these hypotheses for a nesting cuckoo, the white-browed coucal, Centropus superciliosus, by presenting territorial pairs with four types of playback treatment: male–female duets, female solos, male solos, and a heterospecific control. The observed birds reacted aggressively to the conspecific playback by approaching the playback loudspeaker and singing. Pairs responded with duets more often to duet playbacks than to playbacks of female and male solo song. Also, neither sex treated simulated unmated intruders (solo playback) as more threatening than mated intruders (duet playback). Taken together, these findings argue against the mate-guarding hypothesis and instead suggest that duetting in white-browed coucals functions primarily in joint territory defence. In almost all cases, males were the first sex to sing in response to playbacks and, overall, they sang more than females. This suggests that males take a primary role in territorial defence. However, females also responded strongly, especially to the simulation of mated or unmated female intruders, hinting at a sex-specific division of labour in territorial defence.

Summary Numerous studies over the past decade have reported correlations between elevated levels of anthropogenic noise and a rise in the minimum frequency of acoustic signals of animals living in noisy habitats. This pattern appears to be occurring globally, and higher pitched signals have been hypothesized to be adaptive changes that reduce masking by low-frequency traffic noise. However, the sound analysis methods most often used in these studies are prone to measurement errors that can result in false positives. In addition, the commonly used method of measuring frequencies visually from spectrograms might also lead to observer-expectancy biases that could exacerbate measurement errors. We conducted an experiment to (i) quantify the size and type of errors that result from ‘eye-balling’ frequency measurements with cursors placed manually on spectrograms of signals recorded in noise and no-noise conditions, and (ii) to test whether observer expectations lead to significant errors in frequency measurements. We asked 54 volunteers, blind to the true intention of our study, to visually measure the minimum frequency of a variety of natural and synthesized bird sounds, recorded either in noise, or no-noise conditions. Test subjects were either informed or uninformed about the hypothesized results of the measurements. Our results demonstrate that inappropriate methodology in acoustic analysis can yield false positives with effect sizes as large, or even larger, than those reported in published studies. In addition to these measurement artefacts, psychological observer biases also led to false positives – when observers expected signals to have higher minimum frequencies in noise, they measured significantly higher minimum frequencies than uninformed observers, who had not been primed with any expectation. The use of improper analysis methods in bioacoustics can lead to the publication of spurious results. We discuss alternative methods that yield unbiased frequency measures and we caution that it is imperative for researchers to familiarize themselves both with the functions and limitations of their sound analysis programmes. In addition, observer-expectancy biases are a potential source of error not only in the field of bioacoustics, but in any situation where measurements can be influenced by human subjectivity.

The impact of human activity on the acoustic environment is overwhelming, with anthropogenic noise reaching even remote areas of the planet. The World Health Organization has identified noise pollution as one of the leading environmental health risks in humans, and it has been linked to a myriad of short- and long-term health effects in exposed individuals. However, less is known about the health effects of anthropogenic noise exposure on animals. We investigated long- and short-term effects of traffic noise on zebra finches breeding in small communal aviaries, using a repeated measures design. Birds bred in both noise and no-noise conditions, and we measured baseline plasma glucocorticoid levels before, during and after breeding. In addition, we assayed immune function, measured reproductive success and offspring growth and compared rates of extra-pair paternity of breeding adults. Breeding birds had significantly lower baseline plasma corticosterone levels when exposed to traffic noise than when they were not exposed to noise playback. In addition, the nestlings reared during noise exposure were lighter than nestlings of the same parents when breeding in control conditions. Our results suggest that traffic noise poses a more severe hurdle to birds at more vulnerable stages of their life history, such as during reproductive events and ontogeny. While chronic exposure to traffic noise in our birds did not, by itself, prove to be a sufficient stressor to cause acute effects on health or reproductive success in exposed individuals, it did result in disruptions to normal glucocorticoid profiles and delayed offspring growth. However, animals living in urban habitats are exposed to a multitude of anthropogenic disturbances, and it is likely that even species that appear to be thriving in noisy environments may suffer cumulative effects of these multiple disturbances that may together impact their fitness in urban environments.

The ecological novelty of urbanisation poses many challenges to animals. We investigated whether anthropogenic disturbance (artificial light at night and noise) and abiotic factors in cities (temperature and humidity) predict nocturnal activity and rest in free-living urban great tits (Parus major). Our study is the first to relate nocturnal rest in wild birds to levels of noise pollution during the night, an issue that has been shown to be particularly damaging to human health. Unlike previous work on nocturnal behaviour of urban birds, we considered the combined effect of anthropogenic disturbance and urban microclimate to acknowledge that the umwelt of an animal is composed of multiple environmental variables. Using infrared cameras, we observed the nocturnal resting behaviour as a proxy for sleep in 17 birds in nest boxes deployed across the city of Munich, Germany. Although we found marked differences in resting behaviour between individuals, this variation was not related to the measured environmental factors. This finding contrasts earlier studies that reported nocturnal resting behaviour of birds to vary with temperature and light exposure. Although we did not find evidence that urban environmental factors disrupt resting behaviour in great tits, their sleep might still be impaired by the anthropogenic disturbances. To elucidate this issue, further studies are necessary that, for instance, measure brain activity. Urbanisation is a subject of growing concern among scientists, conservationists and policy makers alike. Yet surprisingly, little is known about the impact of urbanisation on wildlife. We investigated whether anthropogenic disturbance (artificial light at night and noise) and microclimate (temperature and humidity) predict patterns of nocturnal activity and resting behaviour in urban great tits (Parus major). Although patterns of resting behaviour differed markedly between individuals, this variation was not related to the measured environmental factors. Our findings are in contrast to previous studies on the effects of urban microclimate and light pollution. At the same time, they suggest that opposing effects of different urban ecological factors may level each other out and thus should be considered in combination.

In the past 100 years, human activities have increased dramatically on both local and global scales, and as a result, levels of anthropogenic noise have risen in many habitats. Elevated noise may have adverse effects for animals, including avoidance, triggering of stress, acoustic masking of cues used to find prey and to avoid predators, and disrupting the exchange of acoustic signals. We focus particularly on the effect on signaling, which has only recently become appreciated, yet may have severe effects. Many species use acoustic signals in survival and reproduction, for example, to avoid predators or to find mating partners. When noise masks these biologically important signals, and thus impairs effective communication, it can negatively affect the animals’ breeding biology and thus the viability of populations. Many species are less abundant near anthropogenic noise sources, but researchers are only just beginning to identify the implications of increasing noise pollution for animal populations. We need to increase our understanding of how anthropogenic noise reduces the maximum distance at which one animal can recognize the acoustic signals of another in its natural habitat. Future studies that link anthropogenic noise with reproductive success are urgently needed to develop reasonable action plans for noise abatement, before the voices of many animals vanish forever.

Background Noise pollution is one of the leading environmental health risks for humans, linked to a myriad of stress-related health problems. Yet little is known about the long-term effects of noise on the health and fitness of wildlife. We experimentally investigated the direct and cross-generational effects of traffic noise on telomeres; a measure of cellular ageing that is predictive of disease and longevity in humans and other organisms. We exposed zebra finches (Taenopygia guttata) to three different treatment groups: 1) parents were exposed to traffic noise before and during breeding, together with their nestling young, 2) fledged juveniles but not their parents were exposed to traffic noise, and 3) control group birds were never exposed to traffic noise. Results Although there was no significant effect of traffic noise exposure at early (pre-fledging) stages of offspring telomere length or loss rate, traffic noise exposure accelerated telomere loss in older (post-fledging) juveniles. Conclusions The age-dependent differences found in this study in telomere loss could occur if parents buffer younger offspring against the detrimental effects of noise exposure and/or if younger offspring are less sensitive to noise exposure. Telomere length during early life has been shown to be positively related to lifespan and the observed noise-induced increase of telomere attrition rate could reduce the fitness of the affected birds and potentially alter the population dynamics of birds in noise polluted areas. Our data highlight the need to consider the developmental stage of an organism to better understand the ecological consequences of anthropogenic change. Electronic supplementary material The online version of this article (10.1186/s12983-018-0275-8) contains supplementary material, which is available to authorized users.

In birdsong, and in most commonly studied acoustic communication systems, research has often focused on temporal and frequency-related signal parameters. However, although variations in amplitude are often overlooked and seldom measured, they are just as critical in communication. Recent studies have demonstrated that vocal amplitude plays an important role in both territorial behaviours and mate choice in birds. Several songbird species have been shown to produce low-amplitude songs, used primarily in aggressive encounters between males. On the other hand, loud songs may be an honest signal of current condition in males and recent studies have shown that females may prefer high-amplitude songs. Although it is generally assumed that louder song is more costly to produce, there is little empirical evidence to support this assumption. Here we review data on the metabolic costs of singing at different vocal amplitudes, and discuss recent studies from our laboratory showing that louder songs elicit stronger aggressive responses from territorial males. Together, these findings suggest that while the energetic costs of singing loudly are negligibly small, social aggression may be a key constraint that limits the upper amplitude of vocal signals. Finally we discuss future directions that can increase our understanding of the role that amplitude plays in acoustic communication in animals.

Sophisticated vocal communication systems of birds and mammals, including human speech, are characterized by a high degree of plasticity in which signals are individually adjusted in response to changes in the environment. Here, we present, to our knowledge, the first evidence for vocal plasticity in a reptile. Like birds and mammals, tokay geckos ( Gekko gecko ) increased the duration of brief call notes in the presence of broadcast noise compared to quiet conditions, a behaviour that facilitates signal detection by receivers. By contrast, they did not adjust the amplitudes of their call syllables in noise (the Lombard effect), which is in line with the hypothesis that the Lombard effect has evolved independently in birds and mammals. However, the geckos used a different strategy to increase signal-to-noise ratios: instead of increasing the amplitude of a given call type when exposed to noise, the subjects produced more high-amplitude syllable types from their repertoire. Our findings demonstrate that reptile vocalizations are much more flexible than previously thought, including elaborate vocal plasticity that is also important for the complex signalling systems of birds and mammals. We suggest that signal detection constraints are one of the major forces driving the evolution of animal communication systems across different taxa.

Anthropogenic noise is one of the fastest growing and most ubiquitous types of environmental pollution and can impair acoustic communication in a variety of animals [1]. Recent research has shown that birds can adjust acoustic parameters of their sexual signals (songs) in noisy environments [2,3], yet we know little about other types of vocalizations. Anti-predator signals contain subtle information that is critical for avoiding predation [4,5], and failure to detect these calls [6,7] as a result of anthropogenic noise pollution could have large fitness consequences by negatively impacting survival. We investigated whether traffic noise impacts both the production and perception of avian alarm calls using a combination of lab and field experiments with great tits (Parus major), a songbird that frequently inhabits noise-polluted environments. In response to experimental noise manipulation in controlled laboratory conditions, great tits increased the amplitude, but not the frequency parameters, of their mobbing alarm calls (hereafter 'alarm calls'). Playback experiments conducted in the wild indicate that current levels of road traffic noise mask alarm calls, impeding the ability of great tits to perceive these critical signals. These results show that, despite the vocal adjustments used to compensate for anthropogenic noise, great tits are not able to restore the active space of their calls in even moderately noisy environments. Consequently, birds are likely to suffer from increased predation risk under noise, with likely effects on their behaviour, populations, and community dynamics in noise-polluted areas.

Animals that use vocal signals to communicate often compensate for interference and masking from background noise by raising the amplitude of their vocalisations. This response has been termed the Lombard effect. However, despite more than a century of research little is known how quickly animals can adjust the amplitude of their vocalisations after the onset of noise. The ability to respond quickly to increases in noise levels would allow animals to avoid signal masking and ensure their calls continue to be heard, even if they are interrupted by sudden bursts of high amplitude noise. We tested how quickly singing male canaries (Serinus canaria) exhibit the Lombard effect by exposing them to short playbacks of white noise and measuring the speed of their responses. We show that canaries exhibit the Lombard effect in as little as 300 ms after the onset of noise and are also able to increase the amplitude of their songs mid-song and mid-phrase without pausing. Our results demonstrate high vocal plasticity in this species and suggest that birds are able to adjust the amplitude of their vocalisations very rapidly to ensure they can still be heard even during sudden changes in background noise levels.

Birds in cities start singing earlier in the morning than in rural areas; commonly this shift is attributed to light pollution. Some studies have suggested that traffic noise has a stronger influence on singing activity than artificial light does. Changes in the timing of singing behaviour in relation to noise and light pollution have only been investigated in the temperate zones. Tropical birds, however, experience little seasonal variation in day length and may be less dependent on light intensity as a modifier for reproductive behaviours such as song. To test whether noise or light pollution has a stronger impact on the dawn chorus of a tropical bird, we investigated the singing behaviour of rufous-collared sparrows ( Zonotrichia capensis ) in Bogota, Colombia at two times during the year. We found that birds in places with high noise levels started to sing earlier. Light pollution did not have a significant effect. Birds may begin to sing earlier in noisy areas to avoid acoustic masking by traffic later in the morning. Our results also suggest that some tropical birds may be less sensitive to variations in day length and thus less sensitive to light pollution.

A growing number of studies ask whether and how bird songs vary between areas with low versus high levels of anthropogenic noise. Across numerous species, birds are seen to sing at higher frequencies in urban versus rural populations, presumably because of selection for higher-pitched songs in the face of low-frequency urban noise, or because birds can avoid masking directly by shifting to higher-frequency sounds (Fernandez-Juricic et al. 2005; Slabbekoorn & den Boer-Visser 2006; Nemeth & Brumm 2009; Gross et al. 2010; Potvin et al. 2010). In addition to changing song frequency, birds are also reported to respond to increased background noise by singing at higher amplitudes (Brumm & Zollinger 2011). Nightingales, Luscinia megarhynchos, for example, sing with a higher sound pressure level in areas with intense traffic noise as compared to quieter locations (Brumm 2004). While frequencyand amplitude-based responses to ambient noise are often considered independently, the twomight also vary in tandem

Effect Sizes and the Integrative Understanding of Urban Bird Song (A Reply to Slabbekoornet al.)Author(s): Erwin Nemeth, Sue Anne Zollinger, and Henrik BrummReviewed work(s):Source: The American Naturalist, Vol. 180, No. 1 (July 2012), pp. 146-152Published by: The University of Chicago Press for The American Society of NaturalistsStable URL: http://www.jstor.org/stable/10.1086/665994 .Accessed: 08/06/2012 07:18

Using real animal noisesin a music piece is fundamentally different from imitating their soundswith an instrument, for imitation with an instrument is an interpretationof the animal voice, whereas biomusic uses a sample of the natural sounditself. Imitating animal voices has a long tradition in Western music: FromVaillant and des Prez in the Middle Ages and Renaissance to Messiaen inthe 20th century, composers have tried to reproduce birdsongs with musicalinstruments (Doolittle). In popular music this technique has been regularlyused since the 1950s (e.g., “Rockin’ Robin” by Bobby Day, which peakedat Number 2 in the US Billboard charts in 1958). Biomusic is a particularform of sampling, but in contrast to reusing a section or sample of a soundrecording from another musical piece, samples from naturalistic recordingsare used. The earliest biomusic piece is probably the symphonic poem

Bird song is a widely used model in the study of sexual selection. Variation in the expression of sexually selected traits is thought to reflect variation in male genetic and/or phenotypic quality. Vocal amplitude is a song parameter that has received little attention in the context of sexual selection, but there is some evidence that the intensity of bird song affects female preferences. Here, we tested whether the amplitude of broadcast song plays a role in male–male competition. We used song playback with varying song amplitude (within the natural amplitude range of the species) and a dummy bird taxidermy to simulate territorial intrusions in the great tit, Parus major, during the fertile period of the female and measured the response of the local male. The results show that playback amplitude significantly affected the subjects’ behaviour: after approaching to within 25 m around the loudspeaker, territorial males stayed longer within that perimeter after the playback of high-amplitude songs compared with low-amplitude songs. Our findings add to the small but growing body of evidence suggesting that vocal amplitude may be a sexually selected song trait.

Birdsong is an important model in the study of evolutionary processes. Vocal amplitude, a song trait that has received little attention to date, varies considerably between individuals, and this variation is important in both female choice and male–male competition. To understand the function of a trait, it is often insightful to look at its origin and ontogeny. Like human speech, birdsong is a learned behaviour, and song amplitude may be adopted from the tutor during vocal ontogeny. However, vocal amplitude may also be bound to song phonetics owing to production constraints. We addressed these ideas with song-learning experiments in the zebra finch, Taeniopygia guttata, a widely used species in vocal-learning studies. The songs of young males were compared to the songs of their genetic fathers and to those of their tutors. We found that the amplitude of tutee song elements was strongly related to the tutor element amplitude, indicating that amplitude is affected by song learning. Therefore, song amplitude could be learned and/or be related to the acoustic structure of song elements owing to production constraints. In line with the latter hypothesis, element amplitude varied with structural element properties (duration and pitch). Mean element amplitude of the genetic father was not related to tutee element amplitude and our heritability estimates of song amplitude were not significant (but statistical power was low). Our findings indicate that the ontogenetic development of adult song amplitude is based on interplay between vocal production learning and physiological constraints.

Food availability is a major evolutionary force that has direct effects on an individual’s body condition. Since sexually selected traits are often condition-dependent, they are likely to reflect food availability or other ecological constraints. Here we test whether bird song, which is thought to be under intense female choice, is sensitive to food availability and might be used by females to assess male body condition and/or territory quality. We manipulated food availability of male zebra finches (Taeniopygia guttata) and assessed the within-individual effects of the treatment on song parameters that are thought to be important in mate choice. We found no effect of food availability on syllable repertoire, proportion of sound versus silence within a song, and mean song frequency. In contrast, treatment birds showed a reduced song rate, an increased latency to sing, and a lowered song amplitude and fundamental frequency. Our study demonstrates that zebra finch song reflects food availability and that songs of well-fed males contain traits that have previously been reported to be more attractive to females. This adds strong support to the general assumption that female song preferences evolved because song reflects male quality and/or territory quality. Moreover, our study provides corroborative evidence for the notion that variation in environmental factors plays an important role in the evolution of mating signals.

Directional communication, in other words the use of body orientation to increase transmission of directional signals, has been widely neglected in studies of animal communication. Here we propose that the experimental analysis of singing direction in birds can provide insights into the function of song. We studied three populations of sedge warblers, Acrocephalus schoenobaenus, in Austria, Britain and Germany. In this species, song is thought to be primarily a mate attraction signal, as males cease to sing once they are paired. Using behavioural observation during playback experiments, we found that males approached a loudspeaker simulating a rival male and also oriented themselves towards the loudspeaker while singing. This finding shows that sedge warbler songs are also directed at rival males and that the dual functions of mate attraction and territory defence were only hidden from the human observer because singing ceases after pairing. We argue that the study of directed communication is an additional tool in addressing functional questions, and that animal orientation should be taken into account when conducting investigations of signalling behaviour.

Abstract On the occasion of the centenary of the discovery of the Lombard effect, we review the literature on noise-dependent regulation of vocal amplitude in humans and other animals. The article addresses the scientific and the biological history of the Lombard effect: first, it sketches the evolution of the study of the Lombard effect, and second it reflects on the biological evolution of the effect itself. By comparing the findings from anurans, birds and mammals, we try to trace back the phylogenetic origins of this basic vocal mechanism for acoustic communication in noise. The current evidence suggests two alternative parsimonious hypotheses: either the Lombard effect is the outcome of a convergent evolution in birds and mammals or it may be a synapomorphy of all amniotes. If the latter is true, then the Lombard effect would have evolved to maintain vocal communication in the presence of noise more than 300 million years ago.

The evolution of bird song is closely related to sexual selection as birds use their songs for mate attraction and territorial defense. Recently, it has been shown that song amplitude can differ markedly between males and that females prefer louder songs. However, it is not known what constrains the production of loud songs. Here, we investigated how interindividual variation in song amplitude affects male‐male territorial interactions. We simulated territorial intrusions by exposing male chaffinches (Fringilla coelebs) to song amplitudes consistent with the high and low end of the distribution of naturally occurring interindividual variation and found significantly stronger territorial behaviors in males exposed to high-amplitude songs. In particular, males sang less, approached the simulated rival closer, and stayed longer in the vicinity when the playback loudspeaker broadcasted louder songs. Our results demonstrate that the intensity of territorial behavior in chaffinches is modulated by the amplitude of songs to which they are exposed, suggesting that social aggression is likely to constrain song performance in this species. We conclude that the variation in song amplitude between males is, at least partly, a result of male‐male competition as probably not all males are able to bear the potential costs of increased social aggression. Key words: bird song, communication, Fringilla coelebs, sexual selection, simulated intruder, song amplitude, song playback, vocal performance. [Behav Ecol]

Song is a fundamental component of territory defense and mate attraction in birds, and androgens (like testosterone) are known to play a key role in controlling it. However, little is known about how differences in testosterone levels between males translate into inter-individual song variation. Indeed, testosterone could affect both the motivation to sing and the structure of song itself. Here, we tested whether experimentally elevated testosterone levels in adult Bengalese finches (Lonchura striata var. domestica), an oscine bird species, have an activational effect on 1) song performance, and 2) song structure. Our results show that testosterone-treated males, in contrast to sham-control males, sang more when confronted with a female. Other performance-related traits, however, such as latency to sing and song amplitude, were not affected. Testosterone-treated males also showed no differences in our two measures of song structure: fundamental element frequency and mean song frequency. Because song structure is known to be organizationally affected by testosterone, our results, synthesized together with findings from the current literature, suggest that in oscine birds, song contains multiple messages about the signaler's hormonal status. First, song performance may reflect current hormonal condition, and second, song structure may reflect the past hormonal state.

When animals colonize cities they often have to adapt their physiology, life history and behaviour to the novel environment. Songbirds rely on acoustic communication for reproduction, and recent studies indicate that songs vary between urban and nonurban habitats. In cities, birds sing louder or use higher frequencies compared to their conspecifics in forests. These habitat-specific differences in song have been interpreted as an adaptation of the city birds to mitigate acoustic masking by low-frequency traffic noise. We compared the songs of blackbirds, Turdus merula, from the city centre of Vienna and the Vienna Woods and found that forest birds sang at lower frequencies and with longer intervals between songs. This difference in song pitch might reflect an adaptation to urban ambient noise. However, the song divergence could also be the result of more intense vocal interaction in the more densely populated city areas or a side-effect of physiological adaptation to urban habitats. We emphasize the need for experimental studies in blackbirds, but also in other species, to clarify a possible causal link between urban acoustics and song characteristics of city birds.

Since acoustic communication is considerably constrained by environmental noise, some animals have evolved adaptations to counteract its masking effects. Humans and New World monkeys increase the duration of brief vocalizations (below a few hundred milliseconds) as the background noise level rises, a behaviour that increases the detection probability of signals in noise by temporal summation. We found that domestic fowl, Gallus gallus domesticus, exhibited the Lombard effect, that is, a regulation of vocal amplitude depending on the background noise level. This vocal mechanism for communication in noise is also found in mammals and other bird species. However, in contrast to primates, the chickens did not regulate the duration of their brief call syllables. This evidence for a lack of regulation of syllable duration may hint at limitations in the degrees of freedom for signal coding. Overall, our findings indicate that the common problem of acoustic communication in noise has led to the evolution of a common solution, the Lombard effect, but also to special adaptations in different taxa.

Bird song is a sexually selected multidimensional signal. A fundamental question regarding the evolution of sexually selected signals is what information they convey and how their honesty is maintained. Song amplitude is a performance-related signal trait that varies considerably between individuals, but this signal dimension has been neglected in past studies. I found that median song amplitude in male nightingales (Luscinia megarhynchos) and zebra finches (Taeniopygia guttata) did not vary significantly with body size or residual body mass. In contrast, I found a significant negative correlation between body size (and also residual mass) and the maximum song amplitude during interactive singing in nightingales. However, the function of these more subtle differences in song amplitude remains to be investigated. By and large, the results of this study suggest that mean song amplitude is unlikely to indicate a bird’s body size or current condition (measured as residual mass).

Several recent studies have tested the hypothesis that song quality in adult birds may reflect early developmental conditions, specifically nutritional stress during the nestling period. Whilst all of these earlier studies found apparent links between early nutritional stress and song quality, their results disagree as to which aspects of song learning or production were affected. In this study, we attempted to reconcile these apparently inconsistent results. Our study also provides the first assessment of song amplitude in relation to early developmental stress and as a potential cue to male quality. We used an experimental manipulation in which the seeds on which the birds were reared were mixed with husks, making them more difficult for the parents to obtain. Compared with controls, such chicks were lighter at fledging; they were thereafter placed on a normal diet and had caught up by 100 days. We show that nutritional stress during the first 30 days of life reduced the birds’ accuracy of song syntax learning, resulting in poorer copies of tutor songs. Our experimental manipulations did not lead to significant changes in song amplitude, song duration or repertoire size. Thus, individual differences observed in song performance features probably reflect differences in current condition or motivation rather than past condition.

Drosophila montana males produce a courtship song that is a prerequisite for mating to occur and which females use to select mates. Here we show that D. montana female responses to courtship song decreased in the presence of high levels of noise within the same frequency band as the courtship song, but not in equivalent noise levels in an adjacent higher frequency band. This suggests that high noise levels overlapping the frequency band of the song impair signal detection, but that a mechanism for frequency filtering exists that reduces the influence of noise in adjacent frequency bands. Although the acoustic environment of this species in the wild is not known, some potential sources of noise are discussed. Nevertheless, our findings show that background noise (biotic or abiotic) within the same frequency band as the courtship song of D. montana can mask it, suggesting that environmental noise might affect mate choice and thereby may influence the evolution of this courtship signalling system.

Short-term adjustments of signal characteristics allow animals to maintain reliable communication in noise. Noise-dependent vocal plasticity often involves simultaneous changes in multiple parameters. Here, we quantified for the first time the relative contributions of signal amplitude, duration and redundancy for improving signal detectability in noise. To this end, we used a combination of behavioural experiments on pale spear-nosed bats (Phyllostomus discolor) and signal detection models. In response to increasing noise levels, all bats raised the amplitude of their echolocation calls by 1.8–7.9 dB (the Lombard effect). Bats also increased signal duration by 13%–85%, corresponding to an increase in detectability of 1.0–5.3 dB. Finally, in some noise conditions, bats increased signal redundancy by producing more call groups. Assuming optimal cognitive integration, this could result in a further detectability improvement by up to 4 dB. Our data show that while the main improvement in signal detectability was due to the Lombard effect, increasing signal duration and redundancy can also contribute markedly to improving signal detectability. Overall, our findings demonstrate that the observed adjustments of signal parameters in noise are matched to how these parameters are processed in the receiver’s sensory system, thereby facilitating signal transmission in fluctuating environments.

Acoustic signals attenuate with the distance over which they travel, but a vocalizing animal might maintain signal transmission by increasing vocal amplitude when addressing a distant receiver. Such behaviour is well known in humans as speakers vary vocal amplitude with changing distance from an audience, a phenomenon that has been interpreted as resulting from our higher cognitive abilities. However, whether nonhuman animals are capable of this form of vocal adjustment appears to be unknown. We investigated whether birds are also able to regulate the amplitude of their vocal signals depending on receiver distance. Male zebra finches, Taeniopygia guttata, increased their song amplitude with increasing distance to addressed females, indicating that songbirds, like humans, respond to differences in communication distance and that they adjust vocal amplitude accordingly. Our findings show that animal communication is flexible in a previously unsuspected way, and that human speech and bird song share a basic mechanism for ensuring signal transmission. We suggest that this behaviour can be accounted for by simple proximate mechanisms rather than by the cognitive abilities that have been thought necessary in humans.

Many animal signals are performed in a highly redundant manner as in some bird species where males sing several renditions of one song type before switching to another. However, differences in signal redundancy between contexts and between individuals are only poorly understood. We found that chaffinches (Fringilla coelebs) in noisier areas (i.e., close to waterfalls and torrents) sang longer bouts of the same song type before switching to a new type, suggesting that they use increased serial redundancy to get the message across in noisy conditions. This is the first evidence of a noise-dependent adjustment of signal redundancy in a songbird. In addition, we found that song types with faster trills were sung in shorter bouts suggesting that the performance of highly redundant song series is probably limited by motor constraints. Thus, in noisy environments, serial redundancy in bird song may reflect a trade-off between successful signal transmission and preventing motor fatigue.

Summary 1. The impact of environmental background noise on the performance of territorial songs was examined in free-ranging nightingales ( Luscinia megarhynchos Brehm). An analysis of sound pressure levels revealed that males at noisier locations sang with higher sound levels than birds in territories less affected by background sounds. 2. This is the first evidence of a noise-dependent vocal amplitude regulation in the natural environment of an animal. 3. The results yielded demonstrate that the birds tried to mitigate the impairments on their communication caused by masking noise. This behaviour may help to maintain a given transmission distance of songs, which are used in territory defence and mate attraction. At the same time, birds forced to sing with higher amplitudes have to bear the increased costs of singing. 4. This suggests that in songbirds the level of environmental noise in a territory will contribute to its quality and thus considerably affect the behavioural ecology of singing

Recent studies have shown that territorial songbirds do not maximize vocal amplitude. Instead, song intensity appears to be a flexible trait that is individually regulated. Given the benefits of singing loudly with regard to signal transmission in the context of territory defence and mate attraction, we investigated whether songbirds adjust the sound level of their territorial songs depending on social influences. While interacting with the playback of a simulated conspecific rival, seven male nightingales, Luscinia megarhynchos, increased the sound level of their songs, on average, by more than 5 dB. The nightingales increased their song level to significantly lower values (about 1 dB) in response to control songs of other species, so the revealed increase in vocal intensity during interactions cannot be fully explained by the acoustical masking of the interfering conspecific songs. Thus, the birds improved signal transmission to a higher degree during conspecific male–male interactions.

Many animal vocalizations are directional, i.e. the sound energy is not emitted omnidirectionally but focused in one direction. Such signal property could be advantageous in order to direct the vocalization towards the addressee, provided its position is known to the sender. On the other hand, a directional signal could be disadvantageous when the position of addressees is not known. Thus, some animals may adjust their behaviour to either counteract or exploit the directionality of their vocalizations depending on the context of communication. We tested this hypotheses in territorial male nightingales (Luscinia megarhynchos) demonstrating the behavioural significance of vocal directionality for the first time. While interacting with the playback of a rival, the birds increased the strength of singing directionality by orientating themselves towards the playback loudspeaker. At the same time, subjects showed less lateral head movements per song during the playback procedure suggesting that nightingales mitigate the directional sound radiation pattern of their songs and emit them in a more omnidirectional way when the position of potential addressees is unclear. During interactions, however, when the position of a addressee is detected, the birds use the directionality of their songs to broadcast them in the direction of the addressee ensuring the most effective signal transmission.

Songs of a Nightingale (Luscinia megarhynchos) were recorded in front of the bird (on-axis) and simultaneously at various off-axis angles. The pattern of sound radiation in the horizontal plane around the singing bird proved to be clearly directional with a median front-back difference (0°–180°) of 4.6 dB. This deviation from omnidirectionality seems to be an inevitable result of the bird's anatomy (i. e. sound shadow effects of the head and body) but some bird species may counteract the directionality of their songs by means of behaviour. Moreover, the directional song radiation pattern might be used to direct songs towards addressees of the signals.

Some animals that use sound to communicate compensate for interference from background noise by adjusting the amplitude of their vocalizations as environmental noise levels vary. Territorial songbirds may have evolved a different strategy, since they can be expected to benefit from maximizing the amplitude of their songs to defend territories and attract females. We tested this hypothesis with calibrated measurements of the song level of male nightingales, Luscinia megarhynchos . All birds increased the sound level of their songs in response to an increase in white noise broadcast to them. A second experiment revealed that noise in the spectral region of their own songs was most effective in inducing the birds to increase vocal intensity. These findings show that nightingales do not maximize song amplitude but regulate vocal intensity dependent on the level of masking noise. The adjustment of vocal amplitude may serve to maintain a specific signal-to-noise ratio that is favourable for signal production. Concurrently, increasing the intensity of songs can maintain a given active space for communication. Thus, vocal amplitude in a territorial songbird can be interpreted as a flexible trait, which is individually regulated according to ecological demands from signal transmission. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved .