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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/.

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.

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.

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

Signal plasticity is a building block of complex animal communication systems. A particular form of signal plasticity is the Lombard effect, in which a signaler increases its vocal amplitude in response to an increase in the background noise. The Lombard effect is a basic mechanism for communication in noise that is well-studied in human speech and which has also been reported in other mammals and several bird species. Sometimes, but not always, the Lombard effect is accompanied by additional changes in signal parameters. However, the evolution of the Lombard effect and other related vocal adjustments in birds are still unclear because so far only three major avian clades have been studied. We report the first evidence for the Lombard effect in an anseriform bird, the mallard (Anas platyrhynchos). In association with the Lombard effect, the fifteen ducklings in our experiment also increased the peak frequency of their calls in noise. However, they did not change the duration of call syllables or their call rates as has been found in other bird species. Our findings support the notion that all extant birds use the Lombard effect to solve the common problem of maintaining communication in noise, i.e. it is an ancestral trait shared among all living avian taxa, which means that it has evolved more than 70 million years ago within that group. At the same time, our data suggest that parameter changes associated with the Lombard effect follow more complex patterns, with marked differences between taxa, some of which might be related to proximate constraints. This article is protected by copyright. All rights reserved.