Songs of the city: noise-dependent spectral plasticity in the acoustic phenotype of urban birds

Experience shapes wild boar spatial response to drive hunts

Human-induced disturbances of the environment are rapid and often unpredictable in space and time, exposing wildlife to strong selection pressure favouring plasticity in specific traits. Measuring wildlife behavioural plasticity in response to human-induced disturbances such as hunting pressures is crucial in understanding population expansion in the highly plastic wild boar species. We collected GPS-based movement data from 55 wild boars during drive hunts over three hunting seasons (2019-2022) in the Czech Republic and Sweden to identify behavioural plasticity in space use and movement strategies over a range of experienced hunting disturbances. Daily distance, daily range, and daily range overlap with hunting area were not affected by hunting intensity but were clearly related to wild boar hunting experience. On average, the post-hunt flight distance was 1.80 km, and the flight duration lasted 25.8 h until they returned to their previous ranging area. We detected no relationship in flight behaviour to hunting intensity or wild boar experience. Wild boar monitored in our study showed two behavioural responses to drive hunts, "remain" or "leave". Wild boars tended to "leave" more often with increasing hunting experience. Overall, this study highlights the behavioural plasticity of wild boar in response to drive hunts.

Comparison of the effects of reef and anthropogenic soundscapes on oyster larvae settlement

Settlement is a critical period in the life cycle of marine invertebrates with a planktonic larval stage. For reef-building invertebrates such as oysters and corals, settlement rates are predictive for long-term reef survival. Increasing evidence suggests that marine invertebrates use information from ocean soundscapes to inform settlement decisions. Sessile marine invertebrates with a planktonic stage are particularly reliant on environmental cues to direct them to ideal habitats. As gregarious settlers, oysters prefer to settle amongst members of the same species. It has been hypothesized that oyster larvae from species Crassostrea virginica and Ostrea angasi use distinct conspecific oyster reef sounds to navigate to ideal habitats. In controlled laboratory experiments we exposed Pacific Oyster Magallana gigas larvae to anthropogenic sounds from conspecific oyster reefs, vessels, combined reef-vessel sounds as well as off-reef and no speaker controls. Our findings show that sounds recorded at conspecific reefs induced higher percentages of settlement by about 1.44 and 1.64 times compared to off-reef and no speaker controls, respectively. In contrast, the settlement increase compared to the no speaker control was non-significant for vessel sounds (1.21 fold), combined reef-vessel sounds (1.30 fold), and off-reef sounds (1.18 fold). This study serves as a foundational stepping stone for exploring larval sound feature preferences within this species.

Changes in bird assemblages following an outdoor music festival: A BACI (before-after-control-impact) monitoring from central Italy

An assessment of the short-term effects of an outdoor music festival (Jova Beach Party event; July 2019; central Italy) on bird assemblages has been carried out, adopting a BACI (Before-After-Control-Impact) survey design, and using the point counts method both in the impact site (Impact, I; where the concert was held) and in comparable Control site (C). In the I site, data have been stratified both for urban (U) and agro-mosaic (M) habitats. When comparing before and after the music event, in IU site, the species richness and the Hill diversity index decreased, differently from CU where species richness a species abundance increased. Diversity profiles highlighted the impoverishment of bird assemblages after the event, but only in the Impact urban habitats. After the musical event, individual rarefaction curves for richness were lower in IU after the concert, while, differently in CU curves are higher. These data suggest an impact in bird assemblages limited to the urban site, due to the stress mainly induced by high intensity noise pollution. Musical events may disrupt the structure of synanthropic bird assemblages, inducing a dispersal of individuals towards the surrounding landscape. Starling (Sturnus vulgaris) appeared a particularly sensitive bird. However, further efforts are necessary to study the effects of these events at species level.

Natural Patterns in the Dawn and Dusk Choruses of a Neotropical Songbird in Relation to an Urban Sound Environment

Urbanization is one of the more important phenomena affecting biodiversity in the Anthropocene. Some organisms can cope with urban challenges, and changes in birds' acoustic communication have been widely studied. Although changes in the timing of the daily organization of acoustic communication have been previously reported, there is a significant gap regarding possible variations in song structure between dawn and dusk choruses. Considering that urbanization imposes different soundscapes for dawn and dusk choruses, we postulate two hypotheses: (i) there are variations in song parameters between dawn and dusk choruses, and (ii) such parameters within the city will vary in response to urban noise. We studied urban and extra-urban populations of Chiguanco Thrush in La Paz, Bolivia, measuring in dawn and dusk choruses: song length; song sound pressure level; minimum, maximum, range and dominant frequency; and the number of songs per individual. The results support our two hypotheses: there were more songs, and songs were louder and had larger band widths at dawn than at dusk in urban and extra-urban populations. Urban Chiguanco Thrushes sing less, the frequency of the entire song rises, and the amplitude increases as compared with extra-urban Chiguanco Thrushes. Understanding variations between dawn and dusk choruses could allow for a better interpretation of how some bird species cope with urban challenges.

Fin whale song evolution in the North Atlantic

Animal songs can change within and between populations as the result of different evolutionary processes. When these processes include cultural transmission, the social learning of information or behaviours from conspecifics, songs can undergo rapid evolutions because cultural novelties can emerge more frequently than genetic mutations. Understanding these song variations over large temporal and spatial scales can provide insights into the patterns, drivers and limits of song evolution that can ultimately inform on the species' capacity to adapt to rapidly changing acoustic environments. Here, we analysed changes in fin whale (Balaenoptera physalus) songs recorded over two decades across the central and eastern North Atlantic Ocean. We document a rapid replacement of song INIs (inter-note intervals) over just four singing seasons, that co-occurred with hybrid songs (with both INIs), and a clear geographic gradient in the occurrence of different song INIs during the transition period. We also found gradual changes in INIs and note frequencies over more than a decade with fin whales adopting song changes. These results provide evidence of vocal learning in fin whales and reveal patterns of song evolution that raise questions on the limits of song variation in this species.

Urbanization drives adaptive evolution in a Neotropical bird

Urbanization has dramatic impacts on natural habitats and such changes may potentially drive local adaptation of urban populations. Behavioral change has been specifically shown to facilitate the fast adaptation of birds to changing environments, but few studies have investigated the genetic mechanisms of this process. Such investigations could provide insights into questions about both evolutionary theory and management of urban populations. In this study, we investigated whether local adaptation has occurred in urban populations of a Neotropical bird species, Coereba flaveola, specifically addressing whether observed behavioral adaptations are correlated to genetic signatures of natural selection. To answer this question, we sampled 24 individuals in urban and rural environments, and searched for selected loci through a genome-scan approach based on RADseq genomic data, generated and assembled using a reference genome for the species. We recovered 46 loci as putative selection outliers, and 30 of them were identified as associated with biological processes possibly related to urban adaptation, such as the regulation of energetic metabolism, regulation of genetic expression, and changes in the immunological system. Moreover, genes involved in the development of the nervous system showed signatures of selection, suggesting a link between behavioral and genetic adaptations. Our findings, in conjunction with similar results in previous studies, support the idea that cities provide a similar selective pressure on urban populations and that behavioral plasticity may be enhanced through genetic changes in urban populations.

Feather corticosterone reveals that urban great tits experience lower corticosterone exposure than forest individuals during dominance-rank establishment

Although the consequences of urbanization for the physiological health of animals are the focus of much active research, an overlooked aspect is how physiology could be indirectly modulated by the urban environment via changes in intraspecific behavioural interactions, particularly among gregarious species. Both urbanization and the establishment, as well as maintenance, of hierarchical rank position are processes that could incur physiological stress. Measurements of glucocorticoids (GCs) in relation to urbanization, however, have yielded inconsistent results. In most cases, GCs have been measured in blood, offering only a 'snapshot' of an animal's current physiological state. Because circulating GCs are incorporated into growing feathers or hair, measurements of feather/hair GCs offer a longer term measure of stress exposure reflecting the whole period of feather/hair growth. During two calendar years, we collected tail feathers from 188 urban and forest great tits (P. major) across multiple sampling sites and analysed corticosterone (CORT-the main GC in birds) levels, reflecting CORT exposure during the extended period in late summer and early autumn when great tits moult and winter flocks are formed. Urban individuals exhibited consistently lower feather CORT (fCORT) levels than forest birds indicating lower overall exposure to CORT during this period. The lower fCORT levels in urban individuals could represent an adaptation to cope with the more challenging urban environment, physiological constraints on stress axis function or a trade-off between the ability to respond to stressors and predation risk during moult. Despite the expectation that CORT responses to urbanization are highly context-dependent, the spatial consistency of our results and agreement with a multi-population study of fCORT in European blackbirds (Turdus merula) suggests a generalization of the effect of urbanization on CORT exposure during post-breeding moult (i.e. not site- or species-specific).

Interactive effects of rising temperatures and urbanisation on birds across different climate zones: A mechanistic perspective

Climate change and urbanisation are among the most pervasive and rapidly growing threats to biodiversity worldwide. However, their impacts are usually considered in isolation, and interactions are rarely examined. Predicting species' responses to the combined effects of climate change and urbanisation, therefore, represents a pressing challenge in global change biology. Birds are important model taxa for exploring the impacts of both climate change and urbanisation, and their behaviour and physiology have been well studied in urban and non-urban systems. This understanding should allow interactive effects of rising temperatures and urbanisation to be inferred, yet considerations of these interactions are almost entirely lacking from empirical research. Here, we synthesise our current understanding of the potential mechanisms that could affect how species respond to the combined effects of rising temperatures and urbanisation, with a focus on avian taxa. We discuss potential interactive effects to motivate future in-depth research on this critically important, yet overlooked, aspect of global change biology. Increased temperatures are a pronounced consequence of both urbanisation (through the urban heat island effect) and climate change. The biological impact of this warming in urban and non-urban systems will likely differ in magnitude and direction when interacting with other factors that typically vary between these habitats, such as resource availability (e.g. water, food and microsites) and pollution levels. Furthermore, the nature of such interactions may differ for cities situated in different climate types, for example, tropical, arid, temperate, continental and polar. Within this article, we highlight the potential for interactive effects of climate and urban drivers on the mechanistic responses of birds, identify knowledge gaps and propose promising future research avenues. A deeper understanding of the behavioural and physiological mechanisms mediating species' responses to urbanisation and rising temperatures will provide novel insights into ecology and evolution under global change and may help better predict future population responses.

Does learning matter? Birdsong-learning program determines coping strategies for living in urban noisy environments

Abstract

Urban noise limits perception by masking acoustic signals, with negative consequences for communication. Although animals relying on acoustic communication are affected, they have developed different strategies to reduce the masking effect of urban noise. Theoretically, birdsong vocal learning confers behavioral plasticity, which may be important for adapting to life in urban environments. To understand the role of vocal learning for adjusting to noisy places, we performed a field study combined with a phylogenetic comparative analysis, comparing passerine species that typically exhibit song learning (oscines) and those that do not (suboscines). Under the premise that vocal learning confers behavioral plasticity, we hypothesized that (1) while oscine species would vary song traits (acoustic parameters), under noisy conditions, suboscines would remain consistent; (2) suboscines may vary birdsong activity in relation to noise; and (3) song learning functions as an exaptation for inhabiting noisy urban environments. We found that oscines only shifted the minimum frequency of their song and did not vary song activity in noisy areas. In contrast, suboscines shifted their complete song upwards and decreased song activity in cities. Our phylogenetic analysis indicated that foraging stratum and song frequency, not learning, best explain adaptation to cities in an evolutionary context. If city noise functions as an ecological filter, frequency traits may serve as an exaptation for colonizing noisy environments. We provided clear evidence that passerine species, depending on their song-learning ability, use different strategies to cope with noise, suggesting that vocal learning determines how birds cope with the masking effect of urban noise.

Significance statement

Since birdsong learning may confer behavioral flexibility, we studied its role for adapting to urban noisy environments. We studied passerines that vary in vocal learning ability combining field data with a phylogenetic comparative analysis. Our methodology may provide information on both the response and the evolutionary advantages of vocal learning for living in noisy urban environments. Although both learner and non-learner birds varied their responses, they displayed different strategies for coping with urban noise. Moreover, differences in vocal learning might not limit colonization of noisy environments, and ecological and acoustic traits may explain adaptation to urbanization. Frequency parameters are conserved evolutionary traits among birds living in cities and may function as a preadaptation that facilitates the colonization of urban environments. Our study suggests that the birdsong-learning program does not help birds colonize cities but determines how they cope with the masking effect of urban noise.

Behavioral plasticity can facilitate evolution in urban environments

Plasticity-led evolution is central to evolutionary theory. Although challenging to study in nature, this process may be particularly apparent in novel environments such as cities. We document abundant evidence of plastic behavioral changes in urban animals, including learning, contextual, developmental, and transgenerational plasticities. Using behavioral drive as a conceptual framework, our analysis of notable case studies suggests that plastic behaviors, such as altered habitat use, migration, diurnal and seasonal activity, and courtship, can have faciliatory and cascading effects on urban evolution via spatial, temporal, and mate-choice mechanisms. Our findings highlight (i) the need to incorporate behavioral plasticity more formally into urban evolutionary research and (ii) the opportunity provided by urban environments to study behavioral mechanisms of plasticity-led processes.

Effect of chronic traffic noise on behavior and physiology of plateau pikas (Ochotona curzoniae)

During the last two decades, numerous studies have shown the effects of traffic noise on animal vocal communication. However, studies on the influences of traffic noise on wildlife behavior and physiology are scarce. In the present study, we experimentally manipulated the traffic noise exposure of plateau pika, a native small mammal widely distributed in the alpine meadow of Qinghai-Tibet Plateau, to explore the effects of traffic noise exposure on its behavior and physiology. We showed that noise exposure increased the pika’s exploration and cortisol concentration (CORT) but decreased the resting metabolic rate (RMR). In addition, the relationships between RMR and exploration or CORT appeared under traffic noise treatment. This study suggests that traffic noise plays a large role in the behavior and physiology of plateau pikas and may have a long-term negative effect on the fitness of rodent populations. Generalizing these non-lethal effects to different taxa is crucial for the conservation and management of biodiversity in this increasingly noisy world.

Invading the soundscape: exploring the effects of invasive species’ calls on acoustic signals of native wildlife

The transmission and reception of sound, both between conspecifics and among individuals of different species, play a crucial role in individual fitness, because correct interpretation of meaning encoded in acoustic signals enables important context-appropriate behaviours, such as predator avoidance, foraging, and mate location and identification. Novel noise introduced into a soundscape can disrupt the processes of receiving and recognising sounds. When species persist in the presence of novel noise, it may mask the production and reception of sounds important to fitness, and can reduce population size, species richness, or relative abundances, and thus influence community structure. In the past, most investigations into the effects of novel noise have focused on noises generated by anthropogenic sources. The few studies that have explored the effects of calls from invasive species suggest native species alter behaviours (particularly their vocal behaviour) in the presence of noise generated by invasive species. These effects may differ from responses to anthropogenic noises, because noises made by invasive species are biotic in origin, and may therefore be more spectrally similar to the calls of native species, and occur at similar times. Thus, in some cases, negative fitness consequences for native species, associated with noises generated by invasive species, may constitute interspecific competition. Possible negative consequences of invasive species calls represent an overlooked, and underappreciated, class of competitive interactions. We are far from understanding the full extent of the effects of invasive species on native ones. Further investigation of the contribution of noise interference to native species’ decline in the presence of invasive species will significantly increase our understanding of an important class of interactions between invasive and native species.

Ruling the roost: Avian species reclaim urban habitat during India's COVID-19 lockdown

As we retreated to our dwellings in the "anthropause" of spring 2020, were the wildlife sightings in previously crowded spaces a reclamation of habitat, or a mere increase in detection? We leverage an increase in balcony birdwatching, a million eBird entries, and difference-in-difference techniques to test if urban avian species richness rose during India's COVID-19 lockdown. Controlling for effort, birdwatchers in the 20 most populous cities observed a 16% increase in the number of species during lockdown. While human activity stopped overnight, and noise and visual pollution decreased soon after, increased species diversity was observed 1-2 weeks later; evidence that gradual population recovery, not better detection, underlay our results. We find at-risk, and rare, species among those reclaiming cities, implying that reducing human disturbance in urban areas can protect threatened species. Increased species diversity likely derives from a reduction in noise and air pollution associated with the lockdown, implying that urban planners should consider conservation co-benefits of urban policies when designing sustainable cities.

Phenotypic plasticity of acoustic traits in high-frequency lebinthine crickets (Orthoptera: Eneopterinae: Lebinthina)

Driven by natural and sexual selection, calling behaviours and call parameters can vary within and between individuals. Phenotypic plasticity can be influenced by environmental conditions (e.g., temperature), size, body condition, and age. Crickets have been classic model organisms for studying the evolution of acoustic communication, but previous studies have focused on field crickets, for which males call at a low frequency, while females exhibit phonotaxis. This study holistically investigated the plasticity of calling behaviours and call parameters across a temperature gradient in a species of lebinthine crickets and examined plasticity between and within individuals. These crickets exhibit a unique communication system, including males calling at a near-ultrasonic frequency while actively searching for females. Ten recording assays at different temperatures were done on males of different sizes and body conditions, half of the assays when the males first became adults and another half 1 month later. Size, body condition, and age group of male crickets, as well as the ambient temperature, had different effects on different calling behaviours (e.g., number of songs produced) and call parameters (e.g., call duration, trill syllable period), even when the acoustic traits were correlated. The crickets also exhibited acclimatisation to the experimental conditions in their calling behaviours and acoustic traits to repeated assays. We also found that calling behaviours were less repeatable than temporal call parameters (e.g., call duration, trill duration), which in turn, were less repeatable than the spectral call parameter (dominant frequency).

Changes to the gut microbiota of a wild juvenile passerine in a multidimensional urban mosaic

Urbanisation is a major anthropogenic perturbation presenting novel ecological and evolutionary challenges to wild populations. Symbiotic microorganisms residing in the gastrointestinal tracts (gut) of vertebrates have mutual connections with host physiology and respond quickly to environmental alterations. However, the impact of anthropogenic changes and urbanisation on the gut microbiota remains poorly understood, especially in early development. To address this knowledge gap, we characterised the gut microbiota of juvenile great tits (Parus major) reared in artificial nestboxes and in natural cavities in an urban mosaic, employing two distinct frameworks characterising the urban space. Microbial diversity was influenced by cavity type. Alpha diversity was affected by the amount of impervious surface surrounding the breeding location, and positively correlated with tree cover density. Community composition differed between urban and rural sites: these alterations covaried with sound pollution and distance to the city centre. Overall, the microbial communities reflect and are possibly influenced by the heterogeneous environmental modifications that are typical of the urban space. Strikingly, the choice of framework and environmental variables characterising the urban space can influence the outcomes of such ecological studies. Our results open new perspectives to investigate the impact of microbial symbionts on the adaptive capacity of their hosts.

Artificial Light at Night Drives Earlier Singing in a Neotropical Bird

Simple Summary

Urban birds have to cope with dominant stressors as anthropogenic noise and artificial light at night by adjusting their song traits. However, evidence of such adjustments has been studied thus far in temperate cities, rather than tropical cities. Here, I tested whether noise and light pollution influence earlier singing behavior in a tropical bird, the Saffron Finch. Birds in highly urbanized sites sang earlier at dawn and this timing difference was driven by light pollution instead of anthropogenic noise. Overall, these results suggest that light pollution could have a detrimental impact on the circadian rhythms of urban tropical birds such as daily singing routines.

Abstract

Anthropogenic noise and artificial light at night (ALAN) can disrupt the morning singing routines of urban birds, however, its influence on tropical species remains poorly explored. Here, I assessed the association between light and noise pollution with the dawn chorus onset of the Saffron Finch (Sicalis flaveola) in a city in Colombia. I studied 32 sites comprised of different conditions of urban development based on built cover. I recorded the time of the first song of the Saffron Finch, the conspecific density and measured anthropogenic noise and ALAN using smartphone apps. The findings of this study show that Saffron Finches living in highly developed sites sang earlier at dawn than those occupying less urbanized sites. Unexpectedly, this timing difference was related to ALAN instead of anthropogenic noise, suggesting that light pollution could drive earlier dawn chorus in a tropical urban bird. Saffron Finches could take advantage of earlier singing for signaling territorial ownership among neighbors. Future studies need to assess the influence of ALAN on the dawn chorus timing of Neotropical urban birds.

Testosterone pulses paired with a location induce a place preference to the nest of a monogamous mouse under field conditions

Changing social environments such as the birth of young or aggressive encounters present a need to adjust behavior. Previous research examined how long-term changes in steroid hormones mediate these adjustments. We tested the novel concept that the rewarding effects of transient testosterone pulses (T-pulses) in males after social encounters alters their spatial distribution on a territory. In free-living monogamous California mice (Peromyscus californicus), males administered three T-injections at the nest spent more time at the nest than males treated with placebo injections. This mimics T-induced place preferences in the laboratory. Female mates of T-treated males spent less time at the nest but the pair produced more vocalizations and call types than controls. Traditionally, transient T-changes were thought to have transient behavioral effects. Our work demonstrates that in the wild, when T-pulses occur in a salient context such as a territory, the behavioral effects last days after T-levels return to baseline.

Urban noise and surrounding city morphology influence green space occupancy by native birds in a Mediterranean-type South American metropolis

Urban green spaces provide natural habitat for birds in urban landscapes, yet the effects of noise and surrounding urban morphology on bird community structure and distribution are not well understood in Latin America, the second most urbanized region in the world. Santiago of Chile is the single city belonging to the Mediterranean ecosystem in South America and is subject to extensive urbanization as seen throughout Latin America. We examined the role of 65 urban green spaces-6 large urban parks (PAR) and 59 small green spaces (SGS)-in harboring native birds during winter 2019, analyzing the quality of green areas in terms of vegetation (i.e. NDVI, native vegetation, and tree cover), exotic bird species, noise levels, and surrounding urban morphology (i.e. building height and cover). Significantly higher noise levels were detected in SGS, along with significantly greater exotic bird (n = 4) richness and abundance than PAR, which possessed significantly greater native bird (n = 25) richness and abundance. Native birds were more abundant than exotic birds in green spaces with average noise levels < 52 dB and average NDVI > 0.5. Occupancy models indicate that green space occupancy by 50% of modeled native bird species was influenced by maximum noise levels, playing a larger role than vegetation (30%) and urban morphology (0%). We stress the importance of developing networks of large green spaces in rapidly urbanizing regions, with abundant tree cover, surrounded by smaller urban morphology, and regulating noise levels to ensure the conservation of native bird communities in cities, particularly those that are threatened.

A comprehensive overview of the effects of urbanisation on sexual selection and sexual traits

Urbanisation can affect mating opportunities and thereby alter inter- and intra-sexual selection pressures on sexual traits. Biotic and abiotic urban conditions can influence an individual's success in pre- and post-copulatory mating, for example through impacts on mate attraction and mate preference, fertilisation success, resource competition or rival interactions. Divergent sexual selection pressures can lead to differences in behavioural, physiological, morphological or life-history traits between urban and non-urban populations, ultimately driving adaptation and speciation. Most studies on urban sexual selection and mating interactions report differences between urban and non-urban populations or correlations between sexual traits and factors associated with increased urbanisation, such as pollution, food availability and risk of predation and parasitism. Here we review the literature on sexual selection and sexual traits in relation to urbanisation or urban-associated conditions. We provide an extensive list of abiotic and biotic factors that can influence processes involved in mating interactions, such as signal production and transmission, mate choice and mating opportunities. We discuss all relevant data through the lens of two, non-mutually exclusive theories on sexual selection, namely indicator and sensory models. Where possible, we indicate whether these models provide the same or different predictions regarding urban-adapted sexual signals and describe different experimental designs that can be useful for the different models as well as to investigate the drivers of sexual selection. We argue that we lack a good understanding of: (i) the factors driving urban sexual selection; (ii) whether reported changes in traits result in adaptive benefits; and (iii) whether these changes reflect a short-term ecological, or long-term evolutionary response. We highlight that urbanisation provides a unique opportunity to study the process and outcomes of sexual selection, but that this requires a highly integrative approach combining experimental and observational work.

Keys to the city: an integrative conceptual framework on avian urban filtering

Urbanization represents a multi-dimensional ecological ‘filter’ for birds determined by a myriad of variables that can change over time. Birds colonising an urban system or staying in a habitat that has been recently urbanised need to overcome both the extrinsic (e.g. food predictability, human activities, and inter-specific interaction) and intrinsic filter variables, ranging from genetic to behavioural changes and/or adjustments. An increasing body of knowledge has identified the behavioural component as crucial for individuals facing the spatiotemporal dynamic urban filters, often after other traits and mechanisms have played their role. Through both developmental (i.e. variability in the expression of genes during ontogeny) and activation plasticity (i.e. alteration of behaviour as a result of individual experience), studies have shown that the identification of cues in novel systems—often determined by extrinsic factors—and learning processes, among other factors, have important impacts on decision-making and innovation. The latter are crucial behavioural traits for thriving in urban settings. Thus, we propose an integrative mechanistic framework based on the process experienced by birds who reach a city and manage to persist in the novel system (becoming urban ‘utilisers’) or those that dwell in an urbanised region who increase their fitness through behavioural responses and adaptations, leading to population persistence (becoming ‘dwellers’). Future field research efforts ought not only to widen the range of focal species, regions, and temporal scales of studies, but also to assess behavioural responses in highly urbanised settings, given that much of our knowledge comes from studies performed in urban greenspaces. Additionally, experimental studies are needed to complement the evidence from field research and to determine causal links.

Territorial Displays of the Ctenophorus decresii Complex: A Story of Local Adaptations

Closely related species make for interesting model systems to study the evolution of signaling behavior because they share evolutionary history but have also diverged to the point of reproductive isolation. This means that while they may have some behavioral traits in common, courtesy of a common ancestor, they are also likely to show local adaptations. The Ctenophorus decresii complex is such a system, and comprises six closely related agamid lizard species from Australia: C. decresii, C. fionni, C. mirrityana, C. modestus, C. tjanjalka, and C. vadnappa. In this study, we analyze the motion displays of five members of the C. decresii complex in the context of their respective habitats by comparing signal structure, habitat characteristics and signal contrast between all species. Motor pattern use and the temporal sequence of motor patterns did not differ greatly, but the motion speed distributions generated during the displays were different for all species. There was also variation in the extent to which signals contrasted with plant motion, with C. vadnappa performing better than the other species at all habitats. Overall, this study provides evidence that members of the C. decresii complex exhibit local adaptations in signaling behavior to their respective habitat, but they also maintain some morphological and behavioral traits in common, which is likely a consequence from the ancestral state.

The duration of artificial light defines sexual signalling in the common glow-worm

Abstract

Artificial light at night is increasing globally, interfering with both sensory ecology and temporal rhythms of organisms, from zooplankton to mammals. This interference can change the behaviour of the affected organisms, and hence compromise the viability of their populations. Limiting the use of artificial light may mitigate these negative effects. Accordingly, we investigated whether the duration of artificial light affects sexual signalling in female glow-worms, Lampyris noctiluca, which are flightless and attract flying males to mate by emitting glow that is interfered by light pollution. The study included three treatments: no artificial light (control), 15 min of artificial light, and 45 min of artificial light. The results show that females were more likely to cease glowing when the exposure to light was longer. Furthermore, small females were more likely to cease their glow, and responded faster to the light, than larger females. These findings suggest that glow-worms can react rapidly to anthropogenic changes in nocturnal light levels, and that prolonged periods of artificial light trigger females to stop sexual signalling. Thus, limiting the duration of artificial light can mitigate the adverse effects of light pollution on sexual signalling, highlighting the importance of such mitigation measures.

Significance statement

Interest in the effects of artificial light at night on animal behaviour has increased in recent years. With evidence for its negative impact accumulating, potential remedies, such as limiting the duration of light exposure, have emerged. To date, however, knowledge on the effectiveness of these methods has remained very limited. We show that female European common glow-worms, which are wingless beetles that glow to attract flying males to mate, responded to prolonged artificial light exposure by discontinuing their glow. Such non-glowing females are not expected to find a mate, making it difficult for them to reproduce. Hence, our study indicates that the duration of artificial light should be limited to protect this night-active beetle and its opportunities for effective sexual signalling. Because many other nocturnal species also need darkness, this study provides valuable information for the development and use of less disruptive night-time lights.

Recreational noise pollution of traditional festivals reduces the juvenile productivity of an avian urban bioindicator

Noise is a pollutant of emergent concern for ecologists and conservation biologists. Recreational noise pollution, especially unpredictable and intermittent sounds, and its effects on wildlife and biodiversity have been poorly studied. Researchers have paid very little attention to the effect of noisy traditional festivals (fireworks and powder-guns). This study aimed to explore the effect of these recreational activities on the juvenile productivity of an urban avian bioindicator: the house sparrow. We studied five pairs of localities in the Valencia Region (E Spain) with noisy traditional festivals. Each pair was composed of one locality with festivals during the breeding season and the closest similar locality, but with festivals outside the reproductive period (controls). Both locality types were sampled twice each spring (May-June of 2019 and 2020). Sampling dates were selected as 15 and 30 days after noisy festivals ended, while the control localities were sampled 1 day after the census of their correspondent town pair with noisy breeding season festivals. The ratio of the juveniles/adults detected during surveys in the influence area of festivals (100-m buffer around the parades route) was used as a house sparrow breeding success proxy. Data were analysed using GLMM: year (2019/2020), festivals season (breeding/non-breeding), survey (15/30 days), and their interactions were included as fixed factors. Pair of localities and locality nested within the pair were random factors. In 2019, juvenile productivity was lower in the towns with noisy traditional festivals during the breeding season than in the control towns. The 2020 festivals were cancelled due to COVID-19. In spring 2020, house sparrow juvenile productivity was the same in both town groups. Lockdown did not increase this species' juvenile productivity in the control localities in 2020 versus 2019.

Pulses of anthropogenic food availability appear to benefit parents, but compromise nestling growth in urban red-winged starlings

The provision of anthropogenic food undoubtedly influences urban bird fitness. However, the nature of the impact is unclear, with both benefits and costs of urban diets documented. Moreover, the influence of short-term fluctuations in food availability, linked to urban weekday/weekend cycles of human presence, is largely unknown. We explored whether breeding red-winged starlings Onychognathus morio in Cape Town, South Africa, altered foraging and provisioning behaviour between days with high human presence (HHP) and days with low human presence (LHP)—i.e. weekdays versus weekends and vacation days. We investigated the relationship between starling diet, adult body mass and nestling development. Breeding adults consumed and provisioned the same quantity of food, but a significantly greater proportion of anthropogenic food on HHP compared to LHP days. Adults apparently benefited from the anthropogenic diet, experiencing significantly greater mass gain on HHP days. However, nestlings experienced a cost, with the number of HHP days during the nestling period associated negatively with nestling size. Adults may, therefore, benefit from the high calorie content of anthropogenic food, while nestlings may be negatively affected by nutrient limitation. The quantity of food available in urban environments may, therefore, benefit adult survival, while its quality imposes a cost to nestling growth.

Multiple constraints on urban bird communication: both abiotic and biotic noise shape songs in cities

Ambient noise can cause birds to adjust their songs to avoid masking. Most studies investigate responses to a single noise source (e.g., low-frequency traffic noise, or high-frequency insect noise). Here, we investigated the effects of both anthropogenic and insect noise on vocalizations of four common bird species in Hong Kong. Common Tailorbirds (Orthotomus sutorius) and Eurasian Tree Sparrows (Passer montanus) both sang at a higher frequency in urban areas compared to peri-urban areas. Red-whiskered Bulbuls (Pycnonotus jocosus) in urban areas shifted the only first note of their song upwards. Swinhoe's White-eye (Zosterops simplex) vocalization changes were correlated with noise level, but did not differ between the peri-urban and urban populations. Insect noise caused the Eurasian Tree Sparrow to reduce both maximum, peak frequency, and overall bandwidth of vocalizations. Insect noise also led to a reduction in maximum frequency in Red-whiskered bulbuls. The presence of both urban noise and insect noise affected the sound of the Common Tailorbirds and Eurasian Tree Sparrows; in urban areas, they no longer increased their minimum song frequency when insect sounds were also present. These results highlight the complexity of the soundscape in urban areas. The presence of both high- and low-frequency ambient noise may make it difficult for urban birds to avoid signal masking while still maintaining their fitness in noisy cities.

Influence of natural and anthropogenic sound sources on the soundscape of the Cananéia estuary, southeastern Brazil

The aim of this study is to characterize the background noise and abiotic and anthropogenic sound sources in the sector with greater anthropogenic use of the Cananéia estuary. The results show that the relative amplitude of background noise decreased with the increase of frequency range, and was higher with greater number of vessels, wind speed and during flood tide. Weekends and vacation periods were shown to be important dates during which background noise increased in the region. The influence of the tide and the wind speed on the relative amplitude was dependent on the frequency range analyzed. Therefore, both abiotic and anthropogenic sound sources were observed to be important factors regarding an increase in background noise in Cananéia. The importance of the continuity of vessel regulation in the region and of future studies that identify whether such noises alter parameters from the sound repertoire of the cetacean species in the region.

What is known - and not known - about acoustic communication in an urban soundscape

Urban environments have some of the most highly modified soundscapes on the planet, affecting the way many animals communicate using acoustic signals. Communication involves transmission of information via signals, such as bird song, between a signaler and a receiver. Much work has focused on the effects of urbanization on signalers and their signals, yet very little is known about how noise pollution affects receiver behaviors and sensory systems. Here we synthesize key findings to date regarding avian acoustic communication in the urban environment and delineate key gaps in knowledge for future work. We leverage our own work comparing current and historical songs from urban and rural habitats for a subspecies of white-crowned sparrows (Zonotrichia leucophrys nuttalli). We use this system, along with findings from other systems, to answer three key questions in the field: (1) Is song variation consistent with temporal and spatial variation in anthropogenic noise? (2) How are birds adjusting their song to the urban environment? and (3) How does song 'urbanization' affect signal function? Our synthesis illustrates that the adjustments birds make to their songs in noisy environments can improve signal detection, but potentially at the cost of signal function. Many key gaps in knowledge need to be addressed to complete our understanding of how acoustic communication systems evolve in urban areas, specifically in regard to sexual selection and female preference, as well as how receivers perceive signals in an urban environment.

Urban Noise Restricts Song Frequency Bandwidth and Syllable Diversity in Bananaquits: Increasing Audibility at the Expense of Signal Quality

Anthropogenic noise can interfere with animal behavior through masking of acoustic communication. In response to masking, animals may change their acoustic signals as an apparent adjustment strategy, but this may have a drawback on signal quality. Songs and calls may show noise-dependent changes in frequency and duration, which may yield some masking avoidance, but may also constrain other acoustic parameters that might carry information about the sender. In the present study, we investigated whether noise-dependent reduction in frequency bandwidth or song duration restricted syllable diversity or song elaboration in a Neotropical songbird, the bananaquit (Coereba flaveola). We show that bananaquits sing higher frequency songs, of narrower bandwidth, in noisier territories, independent of variation in territory density, without significant variation in song duration. We also show that songs with higher minimum frequencies, narrower bandwidths, and shorter durations have on average a lower number of syllable types and higher syllable rates. This finding is in line with an acoustic restriction and may reflect a functional trade-off between audibility and signal value: higher frequencies may be more audible but less elaborate songs may weaken the message of sender quality. Consequently, noise pollution may not only alter avian communities, but also shape acoustic diversity and processes of sexual selection in urban environments.

Say what? On the transmission of acoustic signals in a Neotropical green city

Cities do not only represent noisy systems, but also limit acoustic communication given the complex array of artificial structures through which signals can be trapped or obscured. In this study we performed a field experiment to evaluate the loss of energy of a standardized acoustic emission (generated with notes and a call of the Clay-colored Thrush – Turdus grayi). For this, we emitted the acoustic signal and recovered it at increasing radial distances from 26 fixed emission points (i.e., 10 m, 20 m, 40 m, 80 m, 120 m, 160 m) located across the city of Xalapa (Mexico). Our results show that the emitted signal was not recorded beyond 80 m from the emission point. The number and height of the assessed physical obstacles between the emitted signal and the receiving equipment showed to drive our main result, with built elements representing a major barrier than vegetation ones in terms of the recovered energy of the emitted signal. Interestingly, we found that, for both types of physical obstacles, a height of ~7 m was a common threshold influencing the recovered energy of the emitted signal.

Mechanisms of Cultural Evolution in the Songs of Wild Bird Populations

Young songbirds draw the source material for their learned songs from parents, peers, and unrelated adults, as well as from innovation. These learned songs are used for intraspecific communication, and have well-documented roles for such functions as territory maintenance and mate attraction. The songs of wild populations differ, forming local "dialects" that may shift over time, suggesting that cultural evolution is at work. Recent work has focused on the mechanisms responsible for the cultural evolution of bird songs within a population, including drift, learning biases (such as conformity and rare-form copying), and selection (including sexual selection). In many songs or song repertoires, variability is partitioned, with some songs or song segments being stable and consistent, while others vary within the population and across time, and still others undergo population-wide transitions over time. This review explores the different mechanisms that shape the cultural evolution of songs in wild populations, with specific reference to a long-term investigation of a single population of philopatric Savannah sparrows. Males learn a single four-segment song during their 1st year and sing the same song thereafter. Within this song, the buzz segment is a population marker, and may be stable for decades - variant forms occur but eventually disappear. In contrast, the middle segment is highly variable both within the population and over time; changes in relative prevalence of different forms may be due to cultural drift or a rare-form learning bias. Within the introductory segment, a high note cluster was replaced by a click train between 1982 and 2010, following an S-shaped trajectory characteristic of both selective sweeps in population genetics and the replacement of one form by another in human language. In the case of the Savannah sparrows, this replacement may have been due to sexual selection. In subsequent generations, the number of clicks within trains increased, a form of cultural directional selection. In contrast to the narrowing of a trait's range during directional selection in genetic systems, variation in the number of clicks in a train increased as the mean value shifted because improvisation during song learning allowed the range of the trait to expand. Thus, in the single short song of the Savannah sparrow, at least four different mechanisms appear to contribute to three different types of cultural evolutionary outcomes. In the future, it will be import to explore the conditions that favor the application of specific (and perhaps conditional) learning rules, and studies such as the ongoing song seeding experiment in the Kent Island Savannah sparrow population will help in understanding the mechanisms that promote or repress changes in a population's song.

Noise-Related Song Variation Affects Communication: Bananaquits Adjust Vocally to Playback of Elaborate or Simple Songs

Birds communicate through acoustic variation in their songs for territorial defense and mate attraction. Noisy urban conditions often induce vocal changes that can alleviate masking problems, but that may also affect signal value. We investigated this potential for a functional compromise in a neotropical songbird: the bananaquit (Coereba flaveola). This species occurs in urban environments with variable traffic noise levels and was previously found to reduce song elaboration in concert with a noise-dependent reduction in song frequency bandwidth. Singing higher and in a narrower bandwidth may make their songs more audible in noisy conditions of low-frequency traffic. However, it was unknown whether the associated decrease in syllable diversity affected their communication. Here we show that bananaquits responded differently to experimental playback of elaborate vs. simple songs. The variation in syllable diversity did not affect general response strength, but the tested birds gave acoustically distinct song replies. Songs had fewer syllables and were lower in frequency and of wider bandwidth when individuals responded to elaborate songs compared to simple songs. This result suggests that noise-dependent vocal restrictions may change the signal value of songs and compromise their communicative function. It remains to be investigated whether there are consequences for individual fitness and how such effects may alter the diversity and density of the avian community in noisy cities.

Long-term noise pollution affects seedling recruitment and community composition, with negative effects persisting after removal

Noise pollution can affect species' behaviours and distributions and may hold significant consequences for natural communities. While several studies have researched short-term effects of noise, no long-term research has examined whether observed patterns persist or if community recovery can occur. We used a long-term study system in New Mexico to examine the effects of continuous natural gas well noise exposure on seedling recruitment of foundational tree species (Pinus edulis, Juniperus osteosperma) and vegetation diversity. First, we examined seedling recruitment and vegetation diversity at plots where current noise levels have persisted for greater than 15 years. We then examined recruitment and diversity on plots where noise sources were recently removed or added. We found support for long-term negative effects of noise on tree seedling recruitment, evenness of woody plants and increasingly dissimilar vegetation communities with differences in noise levels. Furthermore, seedling recruitment and plant community composition did not recover following noise removal, possibly due in part to a lag in recovery among animals that disperse and pollinate plants. Our results add to the limited evidence that noise has cascading ecological effects. Moreover, these effects may be long lasting and noise removal may not lead to immediate recovery.

Does anthropogenic noise promotes advertisement call adjustments in the rubí poison frog Andinobates bombetes?

Anthropogenic noise, characterized by higher intensities at low frequencies, can restrict acoustic communication between conspecifics and eventually reduce the fitness of populations. We analysed changes in the call features of 52 males of the poison frog A. bombetes subjected to anthropogenic noise through playback experiments. Thirty-one males did not call during playbacks, but the remaining 21 males did. Fourteen of those 21 males increased their dominant call frequency on average 130.76 Hz when exposed to noise. Males did not increase or diminish the emission rate, number of pulses, and duration of their calls. It is possible that males by increasing the frequency of their calls are showing a behavioural strategy that maintain signal-to-noise ratio, which allows them to communicate acoustically in noisy habitats. Further studies are necessary to corroborate this hypothesis given that the magnitude of the increase in call frequency was small (<100 Hz) for most males.

Noise and landscape features influence habitat use of mammalian herbivores in a natural gas field

Anthropogenic noise is a complex disturbance known to elicit a variety of responses in wild animals. Most studies examining the effects of noise on wildlife focus on vocal species, although theory suggests that the acoustic environment influences non-vocal species as well. Common mammalian prey species, like mule deer and hares and rabbits (members of the family Leporidae), rely on acoustic cues for information regarding predation, but the impacts of noise on their behaviour has received little attention. We paired acoustic recorders with camera traps to explore how average daily levels of anthropogenic noise from natural gas activity impacted occupancy and detection of mammalian herbivores in an energy field in the production phase of development. We consider the effects of noise in the context of several physical landscape variables associated with natural gas infrastructure that are known to influence habitat use patterns in mule deer. Our results suggest that mule deer detection probability was influenced by the interaction between physical landscape features and anthropogenic noise, with noise strongly reducing habitat use. In contrast, leporid habitat use was not related to noise but was influenced by landscape features. Notably, mule deer showed a stronger predicted negative response to roads with high noise exposure. This study highlights the complex interactions of anthropogenic disturbance and wildlife distribution and presents important evidence that the effects of anthropogenic noise should be considered in research focused on non-vocal specialist species and management plans for mule deer and other large ungulates.

Territory Quality Predicts Avian Vocal Performance Across an Urban-Rural Gradient

Human activity around the globe is a growing source of selection pressure on animal behavior and communication systems. Some animals can modify their vocalizations to avoid masking from anthropogenic noise. However, such modifications can also affect the salience of these vocalizations in functional contexts such as competition and mate choice. Such is the case in the well-studied Nuttall's white-crowned sparrow (Zonotrichia leucophrys nuttalli), which lives year-round in both urban San Francisco and nearby rural Point Reyes. A performance feature of this species' song is salient in territorial defense, such that higher performance songs elicit stronger responses in simulated territorial intrusions; but songs with lower performance values transmit better in anthropogenic noise. A key question then is whether vocal performance signals male quality and ability to obtain high quality territories in urban populations. We predicted white-crowned sparrows with higher vocal performance will be in better condition and will tend to hold territories with lower noise levels and more species-preferred landscape features. Because white-crowned sparrows are adapted to coastal scrub habitats, we expect high quality territories to contain lower and less dense canopies, less drought, more greenness, and more flat open ground for foraging. To test our predictions, we recorded songs and measured vocal performance and body condition (scaled mass index and fat score) for a set of urban and rural birds (N = 93), as well as ambient noise levels on their territories. Remote sensing metrics measured landscape features of territories, such as drought stress (NDWI), greenness (NDVI), mean canopy height, maximum height, leaf area density (understory and canopy), slope, and percent bare ground for a 50 m radius on each male territory. We did not find a correlation between body condition and performance but did find a relationship between noise levels and performance. Further, high performers held territories with lower canopies and less dense vegetation, which are species-preferred landscape features. These findings link together fundamental aspects of sexual selection in that habitat quality and the quality of sexually selected signals appear to be associated: males that have the highest performing songs are defending territories of the highest quality.

Sources of individual variation in problem-solving performance in urban great tits (Parus major): Exploring effects of metal pollution, urban disturbance and personality

Despite growing research effort, we have a limited understanding of how urban disturbance factors affect cognitive traits, such as innovative problem-solving. We performed an initial assessment of how metal pollution and urbanization levels on territories are related to problem-solving performance in urban great tits (Parus major), by presenting an obstacle removal test at nest boxes in three urban nest box populations that are exposed to different levels of metal pollution. We predicted that problem-solving capacity might be reduced within the most polluted population due to pollution-related neurological impairments. On the other hand, we predicted that problem-solving might positively correlate with urban disturbance levels on territories, because some past research suggests that problem-solving promotes persistence in urbanized habitats. We also assessed relationships between exploratory personality type, behavioral patterns during tests, and problem-solving performance, and examined the repeatability and fitness correlates of problem-solving. We found no evidence that behavioral patterns or problem-solving performance were related to metal exposure or exploratory personality, or that problem-solving promotes reproductive success. However, birds on territories exposed to more urban disturbance, as quantified by proximity to paths and roads, were more likely to problem-solve. Moreover, an aggressive problem-solving approach negatively predicted problem-solving success, and behavioral patterns during tests and problem-solving success were repeatable. Thus, rather than indicating negative effects of pollution or urban disturbance on problem-solving performance, our study provides preliminary support for the hypothesis that urbanization favors innovative problem-solving, and suggests that problem-solving could be associated with a personality dimension independent of exploratory personality.

Chronic exposure to urban noise during the vocal learning period does not lead to increased song frequencies in zebra finches

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.

Cold Nights, City Lights: Artificial Light at Night Reduces Photoperiodically Induced Diapause in Urban and Rural Populations of Aedes albopictus (Diptera: Culicidae)

As the planet becomes increasingly urbanized, it is imperative that we understand the ecological and evolutionary consequences of urbanization on species. One common attribute of urbanization that differs from rural areas is the prevalence of artificial light at night (ALAN). For many species, light is one of the most important and reliable environmental cues, largely governing the timing of daily and seasonal activity patterns. Recently, it has been shown that ALAN can alter behavioral, phenological, and physiological traits in diverse taxa. For temperate insects, diapause is an essential trait for winter survival and commences in response to declining daylight hours in the fall. Diapause is under strong selection pressure in the mosquito, Aedes albopictus (Skuse); local adaptation and rapid evolution has been observed along a latitudinal cline. It is unknown how ALAN affects this photosensitive trait or if local adaptation has occurred along an urbanization gradient. Using a common garden experiment, we experimentally demonstrated that simulated ALAN reduces diapause incidence in this species by as much as 40%. There was no difference, however, between urban and rural demes. We also calculated diapause incidence from wild demes in urban areas to determine whether wild populations exhibited lower than predicted incidence compared to estimates from total nocturnal darkness. In early fall, lower than predicted diapause incidence was recorded, but all demes reached nearly 100% diapause before terminating egg laying. It is possible that nocturnal resting behavior in vegetation limits the amount of ALAN exposure this species experiences potentially limiting local adaptation.

Urban birdsongs: higher minimum song frequency of an urban colonist persists in a common garden experiment

Environmental changes caused by urbanization and noise pollution can have profound effects on acoustic communication. Many organisms use higher sound frequencies in urban environments with low-frequency noise, but the developmental and evolutionary mechanisms underlying these shifts are generally unknown. We used a common garden experiment to ask whether changes in minimum song frequency observed 30 years after a songbird colonized an urban environment are a consequence of behavioural flexibility. We captured male juvenile dark-eyed juncos, Junco hyemalis thurberi, from two populations (urban and mountain) soon after they reached independence (aged 25-40 days), raised them in identical indoor aviaries and studied their songs at an age of 3 years. We found that the large population difference in minimum frequency observed in the field persisted undiminished in the common garden despite the absence of noise. We also found some song sharing between the common garden and natal field populations, indicating that early song memorization before capture could contribute to the persistent song differences in adulthood. These results are the first to show that frequency shifts in urban birdsong are maintained in the absence of noise by genetic evolution and/or early life experiences.

Acoustic preadaptation to transmit vocal individuality of savanna nightjars in noisy urban environments

As urbanization has expanded dramatically, the impacts of urban noise on wildlife have drawn increasing attention. However, previous studies have focused primarily on diurnal songbirds and much less on nocturnal nonpasserines such as nightjars. The savanna nightjar has recently successfully colonized urban areas in Taiwan. Using 1925 calls recorded from 67 individuals, we first investigated the individual differences of the acoustic structures; and, for those acoustic variables with significant individual differences, we examined the correlation between the acoustic structures and the ambient noise levels. We then compared the transmission efficacy of vocal individuality among three sets of acoustic variables: all acoustic variables, noise-related variables, and noise-unrelated variables. Using seven artificial frequency-shifted calls to represent seven different individuals in playback-recording experiments, we also investigated the transmission efficacy of vocal individuality and variable accuracy in three different urban noise levels (high, medium, low). We found that all 30 acoustic variables derived from the acoustic structures demonstrated significant individual differences, and 14 frequency-based variables were negatively correlated with ambient noise levels. Although transmission efficacy was significantly affected by urban noise, individuality information was still transmitted with high accuracy. Furthermore, the noise-unrelated structures (which included the maximum frequency, the maximum amplitude frequency, and the mean frequency of the call) had a significantly higher transmission efficacy of vocal individuality than the noise-related variables (which included the minimum frequency, the frequency at the start and the end of the call) in both field observation and playback-recording experiments. We conclude that these noise-unrelated acoustic features may be one of the key preadaptations for this nocturnal nonpasserine to thrive so successfully in its newly adopted urban environment.