Dawn singing of the Brownish‐flanked Bush Warbler influences dawn chorusing in a bird community

Despite numerous studies on the function of the avian dawn chorus, few studies have examined whether dawn singing may influence the singing of other species. Here, we built on our previous study which found male Brownish‐flanked Bush Warblers (Horornis fortipes) increase their dawn singing intensity after conspecific playback on the previous day. We reanalyzed those recordings to quantify the start of dawn singing in other nine sympatric songbird species. Ranking‐scaling analyses identified a distinctive sequential pattern of dawn singing among these bird species between the first and the second dawn chorus, and meta‐analysis showed a significant trend to singing earlier in the bird community accompanied by the increase in dawn singing intensity in Brownish‐flanked Bush Warbler. Species with songs most similar to that of the Brownish‐flanked Bush Warbler and species that were phylogenetically distantly related to the Brownish‐flanked Bush Warbler showed a greater shift in the onset of dawn singing. Our study is one of the few studies showing how bird song influences heterospecific singing, and this may influence the temporal organization of song activity in the community, and result in synchronization in singing activities among different species, such as singing in dawn and dusk chorus.     

Phylogenetic and ecological determinants of the neotropical dawn chorus

The concentration of avian song at first light (i.e. the dawn chorus) is widely appreciated, but has an enigmatic functional significance. One widely accepted explanation is that birds are active at dawn, but light levels are not yet adequate for foraging. In forest communities, the onset to singing should thus be predictable from the species' foraging strata, which is ultimately related to ambient light level. To test this, we collected data from a tropical forest of Ecuador involving 57 species from 27 families of birds. Time of first song was a repeatable, species-specific trait, and the majority of resident birds, including non-passerines, sang in the dawn chorus. For passerine birds, foraging height was the best predictor of time of first song, with canopy birds singing earlier than birds foraging closer to the forest floor. A weak and opposite result was observed for non-passerines. For passerine birds, eye size also predicted time of first song, with larger eyed birds singing earlier, after controlling for body mass, taxonomic group and foraging height. This is the first comparative study of the dawn chorus in the Neotropics, and it provides the first evidence for foraging strata as the primary determinant of scheduling participation in the dawn chorus of birds.     

Sleepless in Town – Drivers of the Temporal Shift in Dawn Song in Urban European Blackbirds

Organisms living in urban environments are exposed to different environmental conditions compared to their rural conspecifics. Especially anthropogenic noise and artificial night light are closely linked to urbanization and pose new challenges to urban species. Songbirds are particularly affected by these factors, because they rely on the spread of acoustic information and adjust their behaviour to the rhythm of night and day, e.g. time their dawn song according to changing light intensities. Our aim was to clarify the specific contributions of artificial night light and traffic noise on the timing of dawn song of urban European Blackbirds (Turdus merula). We investigated the onset of blackbird dawn song along a steep urban gradient ranging from an urban forest to the city centre of Leipzig, Germany. This gradient of anthropogenic noise and artificial night light was reflected in the timing of dawn song. In the city centre, blackbirds started their dawn song up to 5 hours earlier compared to those in semi-natural habitats. We found traffic noise to be the driving factor of the shift of dawn song into true night, although it was not completely separable from the effects of ambient night light. We additionally included meteorological conditions into the analysis and found an effect on the song onset. Cloudy and cold weather delayed the onset, but cloud cover was assumed to reflect night light emissions, thus, amplified sky luminance and increased the effect of artificial night light. Beside these temporal effects, we also found differences in the spatial autocorrelation of dawn song onset showing a much higher variability in noisy city areas than in rural parks and forests. These findings indicate that urban hazards such as ambient noise and light pollution show a manifold interference with naturally evolved cycles and have significant effects on the activity patterns of urban blackbirds.     

Experimental evidence for an impact of anthropogenic noise on dawn chorus timing in urban birds

Many animal species are living in urban areas, where they encounter human‐altered environmental conditions. Artificial light and traffic noise are two of the most prominent anthropogenic factors, both of which potentially affect animal life. Here we studied the changes in traffic noise conditions over the morning in the urban bird habitat of the city of Seville, Spain. We tested experimentally whether noise from human activities can cause a shift in the timing of birdsong activity. Our data revealed that noise conditions vary markedly among our replicate set of twelve streets. Relatively quiet streets show low base‐line amplitude levels early in the morning, with frequent events of brief noise bursts, followed by a strong rise in noise levels. Relatively noisy streets have high base‐line amplitude levels from a much earlier start in the morning. Experimental exposure data revealed a noise‐related earlier start of dawn singing for two out of six species: the spotless starling Sturnus unicolor and the house sparrow Passer domesticus. Our experiment did not cover earlier singing species and revealed no impact for species with more‐variable starting times of the dawn singing. Our study provides more insight into the intertwinings of bird and human behavior and confirms the potential for experimental approaches to successfully tackle questions related to the impact of anthropogenic factors on animal life in cities.     

Dawn Singing Intensity of the Male Brownish‐Flanked Bush Warbler: Effects of Territorial Insertions and Number of Neighbors

The dawn chorus is a period of peak singing activity of many songbirds. Numerous studies have sought to understand this widespread phenomenon, and many hypotheses have been proposed to explain the dawn chorus. The social dynamics hypothesis proposes that dawn singing plays an important role in the announcement of territorial occupancy and the regulation of social relationships between males; it predicts that the dawn chorus vocal behavior varies with changes in social relationships. In this study, we tested the influence of territorial insertions and the number of neighbors, on the intensity of the brownish‐flanked bush warbler (Cettia fortipes)'s dawn singing. We found that simulated territorial insertions (playback) caused the males to increase their dawn singing significantly the next day, and males that had many neighbors exhibited more intense dawn singing than did males with few neighbors. Our study provides evidence that dawn singing plays an important role in the announcement of territorial occupancy and the regulation of the social relationships between the males.     

Two explanations of the dawn chorus compared: how monotonically changing light levels favour a short break from singing

I used optimality modelling to compare two of the most plausible and general explanations for the dawn and dusk peaks in bird song output. Kacelnik's explanation is that foraging is inefficient in poor light, but that social interactions are less affected, making singing more worthwhile than foraging. McNamara et al.'s explanation is based on stochasticity in foraging success and overnight energy requirements; it has been extensively analysed with stochastic dynamic programming models. Both explanations are now incorporated into this sort of model. I used various functions to link success of foraging and singing to time of day, but assumed that above some light level there is no further effect. Kacelnik's explanation has as strong an effect as stochasticity in generating dawn and dusk choruses. It also predicts short pauses in the singing output just after the dawn chorus and before the dusk chorus. The former arises because birds delay foraging when it will become more profitable later, until foraging success reaches a plateau, when the energetic debt accumulated makes them forage. The principle of this see-saw double switch in behaviours may apply to other explanations for the dawn chorus, and to other shifts in behaviour when conditions change gradually. The model predicts that from day to day cloud cover determines when a dawn chorus starts, but that overnight temperature and wind strength have more effect on chorus intensity and duration. I discuss what sort of observational and experimental data on singing routines would better test this model. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Song types and their structural features are associated with specific contexts in the banded wren

We studied the use of song types and their acoustic features in different social contexts in the banded wren (Thryothorus pleurostictus), a resident tropical songbird in which males possess about 20 distinctive song types varying in duration, bandwidth, note composition, and trill structure. We recorded six focal males intensively for four days each while we observed context information such as during versus after dawn chorus, presence of the female, counter-versus solo-singing, location at the edge versus centre of the territory, and proximity to the nest. All males used at least some song types differentially during each of these pairs of alternative contexts. Males also preferentially used the song types they shared with a given neighbour when interacting with that bird. Songs delivered during dawn chorus were significantly longer, wider in bandwidth, often compound (double songs), and more likely to contain a rattle or buzz and an up-sweeping trill, compared to songs delivered after dawn chorus. Similar features were also more commonly observed when birds were engaged in intense male-male interactions and boundary disputes after dawn chorus, especially when countersinging at the edge of the territory. The presence of the female caused the male to deliver song types with narrower whole-song and trill bandwidth and fewer rattles and buzzes, and song-type diversity and fraction of compound songs were higher when the female was present. Thus, in addition to using type matching and variations in song-type switching and diversity to signal different levels of aggressive intention, male banded wrens also select song types based on their acoustic structure in different social contexts.     

Bird communities across varying landcover types in a Neotropical city

Urbanization poses a serious threat to local biodiversity, yet towns and cities with abundant natural features may harbor important species populations and communities. While the contribution of urban greenspaces to conservation has been demonstrated by numerous studies within temperate regions, few consider the bird communities associated with different landcovers in Neotropical cities. To begin to fill this knowledge gap, we examined how the avifauna of a wetland city in northern Amazonia varied across six urban landcover types (coastal bluespace; urban bluespace; managed greenspace; unmanaged greenspace; dense urban; and sparse urban). We measured detections, species richness, and a series of ground cover variables that characterized the heterogeneity of each landcover, at 114 locations across the city. We recorded >10% (98) of Guyana's bird species in Georgetown, including taxa of conservation interest. Avian detections, richness, and community composition differed with landcover type. Indicator species analysis identified 29 species from across dietary guilds, which could be driving community composition. Comparing landcovers, species richness was highest in managed greenspaces and lowest in dense urban areas. The canal network had comparable levels of species richness to greenspaces. The waterways are likely to play a key role in enhancing habitat connectivity as they traverse densely urbanized areas. Both species and landcover information should be integrated into urban land-use planning in the rapidly urbanizing Neotropics to maximize the conservation value of cities. This is imperative in the tropics, where anthropogenic pressures on species are growing significantly, and action needs to be taken to prevent biodiversity collapse.     

Birds at the urban fringe: avian community shifts in different peri-urban ecotones of a megacity

Cities can be regionalized in intra-urban and peri-urban areas. The space between urban areas and adjacent systems represents an ecological transition that often acts as a semi-permeable biological filter. In this study, we assessed changes in avian community species richness, density, and composition at different peri-urban ecotones (i.e., urban-croplands, urban-grasslands, urban-shrublands) of northeastern Mexico City. Species richness was lower in the urban component of urban-grassland and urban-shrubland ecotones, while bird densities were higher in the urban components of the urban-grassland and urban-shrubland peri-urban ecotones, mainly due to the high number of urban exploiter species. However, the urban-cropland peri-urban ecotone exhibited a different pattern, with similar low bird species richness and density values between both components (urban and non-urban). A species composition analysis revealed that urban bird communities were not influenced by adjacent non-urban habitats, since the urban components of peri-urban ecotones were more similar among them than in relation to the rest of non-urban components. In summary, results of this study show that urbanization can represent an important biological filter for birds, often reducing species richness and homogenizing avian communities at local scales. As the environmental variables determining ecological processes related to the semi-permeable filter effect that urban areas pose to biodiversity might depend on urban habitats, regions, and spatial scales, further studies are needed to fully understand this phenomenon.     

Early singers attend to conspecific but not heterospecific behavioural cues at dawn

During the dawn chorus songbirds initiate singing activities just prior to sunrise. Environmental factors affect the timing of the dawn chorus, but relatively little is known about how behavioural cues influence chorus timing. We assessed whether early playback of conspecific and heterospecific song influenced chorus start times in two early‐singing temperate species, hermit thrush Catharus guttatus and veery C. fuscescens. We used arrays of GPS time‐synchronized recorders to record the dawn chorus at 30 locations. In each location, thirty minutes prior to natural chorus initiation, we broadcast: 1) an early singing species (hermit thrush or veery), 2) a late singing species (black‐throated green warbler Setophaga virens or Nashville warbler Oreothlypis ruficapilla), 3) a noise control, and 4) also recorded without playback (silent control). Playback treatments were separated by two‐three days at each location. Both hermit thrush and veery sang significantly earlier in response to conspecific playback compared to noise or silent controls. Both species also sang earlier in response to conspecific playback compared to the day before or the day after playback. Neither species sang earlier in response to heterospecific playback (regardless of whether the broadcast species was a naturally early or late song initiator) compared to noise or silent controls. Our results support the theory that the dawn chorus is a communication network where individuals attend primarily to conspecific cues; heterospecific song appears to have minimal impacts on the chorus start times of the two early‐singing species we investigated.     

Experimentally Elevated Plasma Testosterone Levels Do Not Influence Singing Behaviour of Male Blue Tits (Parus caeruleus) During the Early Breeding Season

Sexual selection theory suggests that females might prefer males on the basis of testosterone (T)‐dependent secondary sexual traits such as song. Correlational studies have linked high plasma T‐levels to high diurnal song output. This has been confirmed in experiments where T‐levels were kept high at times when natural T‐levels have decreased. However, surprisingly little is known about the relation between T‐levels during the early breeding season and song. In many passerine birds males sing at a high rate at dawn early in the breeding season, referred to as the dawn chorus. In blue tits (Parus caeruleus), the dawn chorus coincides with the fertile period of the female, whereas diurnal song occurs throughout the breeding season. Previous studies on blue tits showed that characteristics of the dawn chorus correlate with male reproductive success. We experimentally elevated plasma T‐levels in male blue tits during the pre‐fertile and fertile period. Our aim was to test whether increased plasma T‐levels affect dawn song characteristics and increase the amount of diurnal song. Although T‐implants successfully raised circulating T‐levels, we did not find any difference between T‐ and control males in temporal performance measures of dawn song or in diurnal song output. Our results suggest that either there is no direct causal link between song output or quality and individual T‐levels, or experimental manipulations of T‐levels using implants do not permit detection of such effects during the early breeding season. Although we cannot exclude that individual T‐levels are causally linked to other (e.g. structural) song parameters, our results cast doubt on T‐dependence as the mechanisms that enforces honesty on song as a sexually selected trait.     

Cities change the songs of birds

Worldwide urbanization and the ongoing rise of urban noise levels form a major threat to living conditions in and around cities. Urban environments typically homogenize animal communities, and this results, for example, in the same few bird species' being found everywhere. Insight into the behavioral strategies of the urban survivors may explain the sensitivity of other species to urban selection pressures. Here, we show that songs that are important to mate attraction and territory defense have significantly diverged in great tits (Parus major), a very successful urban species. Urban songs were shorter and sung faster than songs in forests, and often concerned atypical song types. Furthermore, we found consistently higher minimum frequencies in ten out of ten city-forest comparisons from London to Prague and from Amsterdam to Paris. Anthropogenic noise is most likely a dominant factor driving these dramatic changes. These data provide the most consistent evidence supporting the acoustic-adaptation hypothesis since it was postulated in the early seventies. At the same time, they reveal a behavioral plasticity that may be key to urban success and the lack of which may explain detrimental effects on bird communities that live in noisy urbanized areas or along highways.     

Degradation of rural and urban great tit song: testing transmission efficiency

Acoustic signals play a fundamental role in avian territory defence and mate attraction. Several studies have now shown that spectral properties of bird song differ between urban and rural environments. Previously this has been attributed to competition for acoustic space as a result of low-frequency noise present in cities. However, the physical structure of urban areas may have a contributory effect. Here we investigate the sound degradation properties of woodland and city environments using both urban and rural great tit song. We show that although urban surroundings caused significantly less degradation to both songs, the transmission efficiency of rural song compared to urban song was significantly lower in the city. While differences between the two songs in woodland were generally minimal, some measures of the transmission efficiency of rural song were significantly lower than those of urban song, suggesting additional benefits to singing rural songs in this setting. In an attempt to create artificial urban song, we mimicked the increase in minimum frequency found several times previously in urban song. However, this did not replicate the same transmission properties as true urban song, suggesting changes in other song characteristics, such as temporal adjustments, are needed to further increase transmission of an avian signal in the city. We suggest that the structure of the acoustic environment, in addition to the background noise, plays an important role in signal adaptation.     

Owl dusk chorus is related to the quality of individuals and nest‐sites

Dawn and dusk choruses represent one of the most investigated topics in avian vocal behaviour, but their underlying basis remains unclear. As with the dawn chorus in passerines, dusk chorus in owls seems to support the mate and rival assessment hypothesis and happens during the most constraining period, as individuals have not yet fed and, under the handicap principle, dusk chorus is likely to reveal inter‐individual differences in competitive ability, body condition and/or habitat quality. Here, a study of vocal displays at dusk of 14 Eurasian Eagle Owls Bubo bubo revealed a temporal succession in the order in which males began their vocalizations. The vocalization order appeared to be related to both the quality of the nesting territory (based upon mean number of fledged young and proportion of rats in the diet) and the male's individual quality, as revealed by haematocrit values and the brightness of the white throat patch.     

Understanding the function of nocturnal song in ovenbirds: males do not countersing at night

Many diurnal bird species vocalize at night, however the function of nocturnal song is, generally, still poorly understood. Previous research has suggested that nocturnal song may serve a social function and is influenced by environmental factors. To test whether males attend to the nocturnal song of conspecifics, we experimentally exposed ovenbirds Seiurus aurocapilla to nocturnal flight songs, and recorded their response both during the night and during the following dawn chorus. We compared latency to song and vocal output before and after playback exposure to determine if males altered their vocalizations in response to exposure to flight songs from an unknown male. We found no evidence of counter singing or change in nocturnal song output, nor a change in vocal output during the dawn chorus following playback exposure. Our results suggest that, in ovenbirds, nocturnal song does not serve as an intraspecific social function. Nocturnal song, through rare, may be significant in the mating systems of some diurnal bird species, and requires additional study.     

Birdsong and anthropogenic noise: implications and applications for conservation

The dramatic increase in human activities all over the world has caused, on an evolutionary time scale, a sudden rise in especially low-pitched noise levels. Ambient noise may be detrimental to birds through direct stress, masking of predator arrival or associated alarm calls, and by interference of acoustic signals in general. Two of the most important functions of avian acoustic signals are territory defence and mate attraction. Both of these functions are hampered when signal efficiency is reduced through rising noise levels, resulting in direct negative fitness consequences. Many bird species are less abundant near highways and studies are becoming available on reduced reproductive success in noisy territories. Urbanization typically leads to homogenization of bird communities over large geographical ranges. We review current evidence for whether and how anthropogenic noise plays a role in these patterns of decline in diversity and density. We also provide details of a case study on great tits (Parus major), a successful urban species. Great tits show features that other species may lack and make them unsuitable for city life. We hypothesize that behavioural plasticity in singing behaviour may allow species more time to adapt to human-altered environments and we address the potential for microevolutionary changes and urban speciation in European blackbirds (Turdus merula). We conclude by providing an overview of mitigating measures available to abate noise levels that are degrading bird breeding areas. Bird conservationists probably gain most by realizing that birds and humans often benefit from the same or only slightly modified measures.     

Why birds sing at dawn: the role of consistent song transmission

The dawn chorus is a widely observed phenomenon. One of the common, but inadequately studied, explanations for the occurrence of the dawn chorus is based on the rationale that atmospheric turbulence, which impairs acoustic communication, is least at dawn, and thus singing at dawn in some way maximizes signal performance. To investigate what possible acoustic benefit is gained through singing at dawn, we transmitted Swamp Sparrow Melospiza georgiana and White-throated Sparrow Zonotrichia albicollis song through open grassland and closed forest both at dawn and at midday. The transmitted songs were re-recorded at four distances from 25 to 100 m. Our results show that the mean overall absolute transmission quality of the signals was not significantly better at dawn than at midday. However, the signal transmission quality was significantly more consistent at dawn than at midday. Also, in general, signal transmission quality decreased with increasing distance. Variability in the transmission quality increased with distance for the White-throated Sparrow song, but not for the Swamp Sparrow song. Consistency in signal transmission quality is a factor that, arguably, is crucial for the identity function of song. This study strongly supports the acoustic transmission hypothesis as an explanation for the existence of the dawn chorus while the demonstration of variability as a key factor in singing at dawn is novel.     

Why do songbirds sing intensively at dawn?

Among the ideas proposed to explain the existence of the dawn chorus in songbirds, the acoustic transmission hypothesis claims that birds sing most intensively at dawn because this is the time of the day when songs suffer least from environmentally induced degradation and hence propagate over the longest distances. In this article, we report on the first sound transmission experiment that directly tests this assumption using natural song from a typically forest-living dawn chorusing bird, the blackcap Sylvia atricapilla. Representative sound elements from the introductory twitter part and from the terminating motif part of the blackcap song were transmitted and re-recorded at three different times of the day: dawn, midmorning, and early afternoon. These recordings were then compared with respect to the following measures of sound degradation: signal-to-noise ratio (SNR), excess attenuation, blurring over song elements, and elongation of song elements by tails of echoes. As could be expected, both the background noise and the SNR varied considerably over the day. More surprisingly the excess attenuation decreased during the day, being lowest in the afternoon. There was no diurnal variation in blurring and elongation by echoes. The results may be explained by the diurnal variation in physical parameters such as temperature, relative humidity, and wind speed. The implications of this for different communication activities are discussed. Overall, the results show that dawn conditions in a temperate deciduous forest do not always constitute the best circumstances for long-range communication and therefore that the dawn chorus cannot be explained by the sound transmission hypothesis. Received in revised form: 24 September 2001 Electronic Publication     

Vocal plasticity in mallards: multiple signal changes in noise and the evolution of the Lombard effect in birds

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 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 yr ago. 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.     

Sex-specific timing of mate searching and territory prospecting in the nightingale: nocturnal life of females

Formal models have shown that diel variation in female mate searching is likely to have profound influence on daily signalling routines of males. In studies on acoustic communication, the temporal patterns of the receivers'' signal evaluation should thus be taken into account when investigating the functions of signalling. In bird species in which diel patterns of signalling differ between males singing to defend a territory or to attract a mate, the diel patterns of mate and territory prospecting are suggested to depend on the sex of the prospector. We simulated newly arriving female nightingales (Luscinia megarhynchos) by translocating radio-tagged females to our study site. The mate-searching females prospected the area mostly at night, visiting several singing males. The timing of female prospecting corresponded to the period of the night when the singing activity of unpaired males was higher than that of paired males. In contrast to females, territory searching males have been shown to prospect territories almost exclusively during the dawn chorus. At dawn, both paired and unpaired males sang at high rates, suggesting that in contrast to nocturnal singing, dawn singing is important to announce territory occupancy to prospecting males. In the nightingale, the sex-specific timing of prospecting corresponded to the differential signalling routines of paired and unpaired males. The temporal patterns in the behaviour of signallers and receivers thus appear to be mutually adapted.