Impacts of Traffic Infrastructure on Urban Bird Communities: A Review

Abstract

With increasing urbanization and related loss of biodiversity, it has become increasingly important to understand the determinants of biodiversity in cities, and to learn how we can maintain existing habitats and improve their quality for both wildlife and humans. Detrimental effects of urbanization on animals such as noise and light pollution, have frequently been reported, but comparatively little is known about the connection between different types of traffic infrastructure and their impacts on urban birds. Here, we provide an overview of the existing knowledge about bird responses to traffic-related stressors, and most importantly, we highlight that this aspect has not been satisfactorily investigated in urban environments. Therefore, we suggest suitable study systems and designs with which the effects of traffic infrastructure on bird communities in cities could be studied, and how biodiversity, in tandem with human wellbeing, in cities would benefit from improvements to the existing infrastructures. In doing so, we aim to strengthen the connection between human wellbeing and birds through research that will ultimately facilitate the development of sustainable cities.

1. Introduction

With the abandonment of rural areas and the growth of urban human populations, and thus, expansion of cities and their surroundings, developing sustainable and liveable urban environments for people and wildlife has become a major challenge. Urbanization is causing drastic environmental changes and decreases in biodiversity globally, as the highest rate of urban growth is expected to be in regions with high biodiversity [1,2,3]. The interface between city residents and birds is highly relevant, since in urban environments, birds are potentially our closest relationship to nature [4,5]. This link has been made even clearer in recent macroecological studies which have shown that bird diversity was a good indicator of human wellbeing, similar to the effect of income [6], and that it increased mental health in time-lasting manner [7,8]. Furthermore, birds are among the most suitable indicators for monitoring the effects of land use conversion on biodiversity [4,5,9,10]. In urban bird ecology, species are classified according to how they cope with or even take advantage of urbanization effects, such as human population density. The so-called urban exploiters typify cities worldwide, and show how urbanization is leading to the homogenisation of urban biodiversity, for example, the house sparrow Passer domesticus, common starling Sturnus vulgaris, and common blackbird Turdus merula [11,12,13]. To investigate the effects of environmental changes, including urbanization, on biodiversity, birds have been monitored at large scales and across long time series within standardized monitoring schemes and citizen-science projects [14,15]. More detailed relationships between birds and their urban environments have been investigated in studies using focal species and experimental designs [16,17]. These studies have shown that birds provide excellent model taxa to understand which effects urbanization has on wildlife, both in terms of biodiversity and on the population level of species. One of the impacts of urbanization is the proliferation of road networks. If accompanied by increasing human population size, increasing traffic load is a further effect of urbanization. In fact, car traffic and grey infrastructure, including all artificial structures and buildings, have been associated with decreased species richness, evenness, and diversity [18,19] as well as an increase in areas with impermeable surfaces and light pollution, which have been shown to be important drivers for biodiversity loss in cities [2]. The diverse effects of increasing human population sizes and traffic loads have ultimately led to a decrease in biodiversity along traffic infrastructures (linear structures for the transportation of people and goods between places, including airports, highways, railways, major and minor roads, and bicycle and pedestrian paths) and in their surroundings through noise, light, and air pollution, as well as habitat degradation and fragmentation [5,20,21,22]. These impacts have led to the fact that urban areas usually have low structural plant and insect diversities, and thus, provide fewer nesting and foraging sites for birds that avoid low-quality habitats around traffic infrastructures [23,24].

A meta-analysis on birds and mammals, using mostly data from North America and Europe, indicated that in a one-kilometre zone around traffic infrastructures, the effects on bird populations were significant, whereas for mammals, the size of this zone was up to five kilometres [22]. This distance has been widely used to assess the broader impacts of roads and railways (the ”effect zone”) on bird populations. The relevance of this threat to bird diversity was highlighted in a nation-wide study conducted in Sweden that showed a significant proportion of protected areas existed inside the effect zone of traffic infrastructure and that, for urban areas, this proportion was even higher [25]. Few studies have reported beneficial effects of traffic infrastructures on birds that included higher availability of food resources from road kills, provision of perches for hunting, warm surfaces for thermophilic species, and alternative nesting structures [26]. These positive associations with roads and train tracks are, however, highly species specific and are not assumed to favour overall bird diversity. Moreover, beneficial effects have mostly been found in low-traffic and wooded road habitats [26]. This highlights how important it is to plan road construction with regards to human mobility and also with respect to the impacts on adjacent habitats for wildlife that will be affected by disturbances related to infrastructure. On a smaller scale, bird responses to disturbances due to traffic, for example, local breeding bird density, have been investigated in woodlands and agricultural landscapes [27]. Whether or not these effects of disturbances due to traffic infrastructure are found at similar magnitudes and directions in urban habitats has not been reviewed so far. Here, we focus on which and how traffic-related stressors impact urban bird diversity, their abundance, and reproductive success, and we provide advice on how to achieve higher biodiversity in cities and also to improve citizens’ wellbeing.

2. Aims and Methods

We need to develop urban areas sustainably in order to accommodate growing urban populations, which has been stated as Envision2030 Goal 11 by the United Nations [28]. Road transport is clearly a key feature and we need to plan new settlements in a way to minimize impacts on biodiversity, which in turn, will have benefits for the human population. We need to better understand the impacts of road infrastructure on biodiversity within urban areas to inform such plans. Therefore, we conducted a qualitative review on impacts of traffic infrastructure in urban areas by summarizing the different traffic-related stressors on urban birds and discussing their relevance for urban birds (Section 3). Furthermore, we propose possible designs for empirical studies and large-scale data analyses to address the current gaps in knowledge about which traffic infrastructures have the strongest effects (Section 4). Finally, we provide suggestions for how urban traffic infrastructures could sustain healthier and more diverse bird communities, and we also discuss how humans would benefit from these improvements (Section 5).

First, we conducted a broad literature search on GoogleScholar with the search terms: “urban bird diversity”, “conservation”, “biodiversity”, “urban bird”, “urban traffic”, “noise pollution”, “air pollution”, “artificial light”, “traffic infrastructure”, “human wellbeing”, “mental health”, “human health”, “urban vegetation”, “greenspaces”, and “street design”. The abstracts of relevant titles of peer-reviewed papers (primary research articles and reviews) in English language were checked. Next, further studies in the literature were found by checking the references in the downloaded papers that were relevant to the research question. Finally, we used ResearchGate, to search, using the above-mentioned terms, and specifically search for published studies in the literature by previously identified authors in the field of urban bird ecology and human wellbeing. We supplemented the collection with articles recommended by colleagues. As a result, we found 139 relevant papers (and few project websites), of which we cited 98 papers.

3. Impacts

Traffic disturbs animals directly, for example, by road kills, and also indirectly as it produces additional noise, light, and air pollution. Among birds, this can lead to lower nest success and survival, higher stress levels, and a shift in daily rhythms [29,30,31]. In addition to these stressors, traffic infrastructures themselves lead to habitat degradation and fragmentation, as they increase the percentage of impervious surface in a city, and thus, lead to the removal of green spaces, i.e., tree cover and connectivity between habitats. In the following sections, we decribe the most relevant traffic-related stressors on urban birds that we identified in the existing literature and point out where knowledge on their effects on birds in urban settings is lacking.

3.1. Noise

Most studies have shown negative effects of traffic on birds through anthropogenic noise, but not all species or guilds are affected in the same way [27]. A study conducted in Australia showed that the detection probability of the target species decreased significantly with increased traffic noise and volume, where the effect of noise was considered to be the more important driver of this decrease [20]. Similar results, with an emphasis on the importance of noise, have been found in the Brazilian metropolitan city, Belo Horizonte, where parks with higher noise levels had lower species diversity and richness [31]. To understand the mechanisms behind the traffic effects, an experimental setting, using a “phantom road” with playback of traffic noise at a common stopover site in southern Idaho (USA), showed that, in general, bird abundance declined by one quarter and that some species avoided the fake road completely [16]. Furthermore, in a natural experimental setting of streets with different noise levels in the city of Sevilla (Spain), two out of six species shifted their song activity to earlier times in the morning, avoiding the noise of “rush hour” [32]. Similar results have been found for European robins Erithacus rubecula that sing during night to avoid day traffic noise [17]. A recent study conducted at a closed airport in Berlin (Germany) showed that birdsong in a highly noise-polluted environment shifted to earlier times so that air traffic noise effects were experienced in a shorter interval of the dawn song. Even after closure of this airport, some species did not immediately return to the rhythms of conspecifics in control populations, showing that noise pollution can have long-lasting effects [33].

Looking at the consequences of noise on bird breeding behaviour and performance, several studies around the globe have investigated noise effects on single target species. For example, in European serins Serinus serinus, singing activity was found to have increased spatially and temporally when noise levels increased, but only up to a threshold of 70 dB, after which they stopped singing. This was assumed to imply that behavioural adjustments to high noise levels were possible, but costly [34]. Further negative impacts of noise were found on the hunting success of northern saw-whet owls Aegolius acadicus, which decreased with increasing noise levels [35]. With respect to physiological responses to noise, European starlings, considered to be urban exploiters, did not show any stress response to traffic noise and their fledging success was not impacted in environments with higher traffic noise [36]. The same result was found in a common urban breeder, the house sparrow [37]. On the contrary, a study on caged Sunda zebra finches Taeniopygia guttata showed that noise led to the suppression of immune function, which is a stress response, and inaccuracies in vocal learning, and thus, was a significant stressor that limited post-hatching development [38].

We conclude that some species may have the capability to adapt and/or show higher resilience to noise effects than others, but overall, species diversity and abundance is decreased in noisy environments. For birds, especially species with decreasing population trends, the overall detrimental effects of noise pollution on urban bird communities should be considered when designing traffic infrastructure. So far, the noise effects of different traffic infrastructures on bird diversity and reproductive success have not been investigated in urban environments, even though noise has been detected as a major stressor for birds in cities.

3.2. Artificial Light

Light pollution is a global phenomenon and it is considered to be an important threat to biodiversity worldwide [39]. A temporal shift in behaviour is among the most reported direct effects of light pollution on birds, driven by the impacts on avian circadian and seasonal rhythms [40]. It can advance breeding (e.g., lay dates) and moulting in urban populations by a month, as studies on common blackbirds have shown [41,42], but it also has impacts at a daily level, since urban birds have been reported to advance their singing activity to earlier hours, especially the early starting singers, for example, common blackbirds and European robins [40,43]. To start singing early (on the seasonal and day level) has effects on breeding schedules, i.e., lay date. Whether these effects are detrimental to the reproductive success and fitness of urban birds has not yet been fully understood and differs between species and environments [44]. For example, in more northerly located cities, earlier breeding might be disadvantageous when environmental conditions, such as temperature and precipitation, are still unfavourable for successful breeding. Furthermore, artificial light at night (ALAN) has been found to alter sleep behaviour and likely led to neuronal death in birds, and thus, it is an important threat to the health of birds [45]. Again, these effects differ between species and nesting guilds, as cavity breeders are less exposed to light pollution during night and differences in sleep behaviour have not been shown in an experimental setting with different treatments of light [46]. ALAN also affects birds in cities and their suburbs indirectly. Although providing locally, and in the short-term, higher invertebrate food resources for some species by attracting insects at lighting structures, the long-term decrease in insect abundance in cities caused by ALAN impacts birds negatively [47].

A continent-wide study in the USA showed high exposure to ALAN by avian migrants above cities, especially in autumn, by using remote-sensing and radar data across a period of 22 years [48]. Negative effects on seabirds and nocturnally migrating birds have been found in numerous studies, which have confirmed ALAN to be a major threat, since birds were attracted by bright light sources which led to collisions with artificial structures or selection of low-quality stopover habitats [49,50,51].

Previous literature reviews have shown that there is a broad body of knowledge on ALAN and its various effects on birds [52,53,54], but we found that studies about the specific contributions of different types of urban traffic infrastructure to behavioural changes in singing activity, breeding success, and overall bird diversity due to light pollution were missing. It is to be expected that roads and other linear traffic infrastructures, with streetlights that are less bright or in which lighting is switched off periodically, would result in fewer collisions with human infrastructure [50]. This would also support maintenance of the natural rhythms of bird singing activity and, as a consequence of the latter, would have less influence on their breeding phenology and success. However, other light sources such as buildings with glass facades [55] and other factors linked to traffic such as noise, would diminish the beneficial effect of a dark road.

3.3. Road Kills

Many studies have shown that the impact of road kills on the overall survival of birds was small to negligible; thus, road kills do not influence populations significantly [27]. However, for some species, adult mortality is affected severely by road kills. A study on Lanzarote, Canary Islands (Spain), showed that bird road kills were most frequent on roads with high speed limits and low traffic volumes and among urban and nocturnal bird species [56]. Clear effects on populations have been found in owls, i.e., Strigiformes [57] and the Florida scrub jay Aphelocoma coerulescens. In the latter, individuals with breeding territories along roads have shown significantly higher adult mortality during breeding seasons due to car traffic, and therefore, are assumed to be a sink population that survives only by immigration from other populations [58]. Contradictory to this, a study conducted in The Netherlands found that minor roads may have a higher mortality impact on wildlife than major roads as they are frequented more by wildlife and usually within more natural habitats than major roads [59]. We did not find studies on bird mortality due to road kills inside cities, nor mortality impacts of different types of traffic infrastructure such as tram or train tracks, roads in residential areas, roads in the city centre, and bike lanes. All of these different modes of transport could have effects, most likely differential, and would be worthy of consideration.

3.4. Direct Disturbance by Traffic

No effects of pedestrian traffic rate on bird diversity in green spaces were found in Melbourne (Australia) [60], whereas in a comparative study between wooded and non-wooded streets in Madrid (Spain), pedestrian and car traffic rate lowered species richness, but this effect was larger and observed in more species [23]. Whether or not these effects are consistent across cities, countries, and continents and whether or not different modes of transport (car, bike, tram, or within-city train) have differential effects, have not been investigated.

3.5. Degradation and Loss of Foraging and Nesting Habitat

A study conducted in Madrid (Spain) showed beneficial effects of vegetation structures along roads, especially trees, on bird species richness and the presence of different foraging and nesting guilds. The presence of different bird species depended on their breeding and foraging requirements, for example, in non-wooded streets, the number of species recorded was close to zero, whereas 56% of the urban park species richness and four out of eight foraging guilds were also found in wooded streets, related to the cover and structure of grass patches, shrubs, and trees [23]. In the city of Mar del Plata (Argentina), species and functional diversity were found to be higher in the 3 m narrow zig-zag street type called “chicane”, with higher and more diverse vegetation structure as compared with ordinary roads (10 m wide) [61]. Low quality roadside habitats lead to a decrease in bird density [24], and also breeding success, as a study conducted in The Netherlands on willow warblers Phylloscopus trochilus showed: In habitats along roads with high traffic volumes, the number of territories occupied and of successfully breeding males were significantly decreased relative to territories far from roads, most likely driven by the disturbance effect of noise, showing that roadside habitats were inhabited by sink populations [62]. Whether these observations are also true within cities is still unknown.

3.6. Fragmentation

Linear habitat structures in cities can benefit the connectivity between populations when used as urban greenways. This was found in a multi-taxon study conducted in Birmingham (UK). For small mammals, for which dispersal is crucial for a sustainable population, the importance of the suitability of linear structures as corridors was highlighted, whereas for plants, butterflies, and beetles, the quality of the actual habitat was more important and the corridor function of an urban greenway was negligible [63]. Therefore, the suitability of urban structures is taxon specific and needs to be investigated separately. Fragmentation acts on population connectivity in birds at a behavioural level rather than by limiting their mobility and it is species specific, as even highly urbanized patches can be crossed successfully by some species [29]. In general, roads can function as corridors for birds [27], but the barrier effect for wildlife is higher in bigger and wider roads [64]. Furthermore, the beneficial effect for bird species richness is higher when streets are connected to parks [23]. In urban habitats, little is known about which traffic infrastructures can function as corridors for birds.

3.7. Air Pollution

Since the potential negative physiological effects of air pollution on birds’ health have been recently reviewed by Diáz et al. 2022 [5], we have not discussed this topic here. To the best of our knowledge, no empirical studies in situ have yet investigated the direct relationships among urban traffic, air pollution, and bird responses, such as hatching rate of eggs, chick development, and juvenile or adult mortality. A study investigating air pollution effects on cyclists showed that air pollution associated with bike lanes along low-traffic routes was much lower than that of bike lanes along high-traffic routes, and therefore, cyclists’ exposure to air pollution depended on the bike lane they chose [64]. The long-term effects of bike route choice have not been investigated so far. However, according to these explorative study on humans, we expect that the same structures with low air pollution exposure, for example, bike lanes separated from high-traffic routes, would also have beneficial effects for birds.

4. Research Priorities and Study Designs

Although noise has been detected as a major stressor for birds in cities, noise effects of different traffic infrastructures on bird diversity and reproductive success have not been investigated in urban environments. This could be addressed, for example, by comparing the effects between major and minor roads, as well as tram and train tracks and bike lanes, and should be in combination with further environmental variables, such as air pollution, artificial light, and adjacent vegetation structures (see

Table 1. Overview of different study designs suitable for targeting the effects of traffic infrastructure in urban habitats on different aspects of bird ecology. Reproduction includes demographic variables, e.g., clutch size, and also other life-history parameters, e.g., lay date. The contribution gives simplified information on how suitable each type of study design is for the investigation of causal relationships (i.e., mechanisms behind the observed patterns in bird responses), and conservation matters (i.e., diversity, population trends, and distribution). The spatio-temporal scale of each type is based on the studies reviewed. The personnel, time, and cost efforts for data collection are simplified and estimated by the authors, based on the qualitative review of the methods in the literature. * Although, it should be noted that bird citizen-scientists are often highly experienced, which is exceptional in citizen-science projects.

Type	Target	Contribution	Spatio-Temporal Scale	Personnel	Time Effort	Costs	Examples
Focal species	Behavioural and physiological responses, reproduction	Highly suitable for causal relationships	Small: local, 1–3 years	Highly trained	High	High	House sparrow [37], European starling [36], blue tit [65,66,67], Florida scrub jay [58]
Experimental design	Behavioural and physiological responses, reproduction	Highly suitable for causal relationships	Small: local, 1–3 years	Highly trained	High	High/Very high	Noise [16,20,32,35]; light [36,37,66]
Bird diversity survey	Species diversity, richness, distribution, population trends	Possibly suitable for causal relationships	Medium: regional, local, 1–3 years	Trained	Medium	Low/Very low	Line transects [23,60,61,68]; point counts [16,31,68,69,70]
Standardized national monitoring programme	Species diversity, richness, distribution, population trends	Suitable to identify trends and large-scale drivers	Large: national, 15–56 years	Less trained	Medium	Very low	Europe (PECBMS, [71]), North American Breeding Bird Survey [14]
Citizen-science	Species diversity, richness, distribution, population trends	Suitable to identify trends and large-scale drivers	Very large: global, national, 10–20 years	Not trained *	Very low	Very low	eBird (9 years, global) [2] Ornitho (19 years, Europe) [15]

, Section 4.1, and Section 4.2). For ALAN, the specific contributions of different types of urban traffic infrastructures and their lighting types would inform city planners how to avoid or reduce behavioural changes in singing activity, breeding success (see Section 4.1 and Section 4.2), and overall bird diversity (see Section 4.3, Section 4.4 and Section 4.5) due to light pollution. Furthermore, it remains to be investigated how different urban traffic infrastructures contribute to direct disturbance (see Section 4.1) or road kills (see Section 4.5), how they may serve as sustainable breeding habitats, and how breeding success and territory occupation vary within and across species (see Section 4.1). Future studies should also target the connectivity between urban habitat patches by analysing how different categories of roads within city train and tram tracks, bike lanes, and pedestrian pathways allow movements of birds, to provide advice on how to improve habitat connectivity in urban landscapes (see Section 4.3).

Here, we summarize study designs and focal species that have been used in (urban) bird ecology and would be suitable for future studies on the effects of traffic infrastructure in urban environments. We list potential studies according to the estimated efforts needed to carry out the respective data collection, from high to low effort in terms of how trained the personnel need to be and how time consuming and costly the data collection was (Table 1).

4.1. Focal Species

Studies on typical urban birds allow the behavioural and physiologic responses to environmental factors to be addressed in detail, such as noise and light, and allow the investigation of detailed species-specific effects of these, i.e., on reproductive success, survival, and fitness. This type of study may provide insights into how phenotypic plasticity in species or guilds allows them to adjust to environmental changes and different intensifications of urbanization and its stressors, i.e., noise and light pollution. Observations on populations and/or individuals of a single species breeding along a traffic infrastructure gradient from high to low disturbance (e.g., from major roads to minor roads to bike lanes) would shed light on the combined effects of noise, air, and light pollution; road kills [58,67]; habitat degradation and fragmentation on the productivity, and thus, prospected population trends when certain infrastructures increase. To understand the effects of different threats on species density, reproductive success, and thus, productivity, it is crucial to carry out a detailed monitoring of territories and/or nests, habitat measurements, and robust statistical modelling. Spatial and temporal scales are usually low, for example, local installation of nest boxes. Despite some longer time series in monitoring these (see, e.g., [37]), specific research projects have usually been carried out within a short time period or even within a single breeding season [58]. Therefore, the long-term effects of traffic-related stressors have rarely been investigated. The time effort is high as daily field work must be carried out throughout one/several breeding seasons (2–4 months/year). The personnel need to be trained in ecological field methods, such as nest searching and monitoring, (chick) ringing, resighting of ringed individuals, installation of additional technical equipment (e.g., to measure noise, light, temperature, and precipitation). The costs depend on the choice of additional technical equipment, for example, data loggers, but could be low, for example, by using already installed nest boxes.

Being typical urban birds [13] and cavity breeders, suitable model species are the house sparrow [37], European starling [36], and blue and great tit [65,66,67], for which nests can be monitored with constant effort, which includes the collection of data on nestling condition and growth. Locations can be selected along an environmental gradient (e.g., noise and light), can be repeatedly used once installed, and environmental variables such as temperature and precipitation can be measured at the same locations.

4.2. Experimental Design

Experimental design research aims to investigate causal relationships between stressors and bird responses, either behavioural, physiological, or regarding demographics, such as nest success, under (more or less) controlled conditions. They allow the impact to vary gradually, for example, by the placement of a stressor and choosing its level, and they analyse its effect on bird responses as a continuous variable. For example, for reproducing distinct levels of anthropogenic noise, recorders have been used to simulate traffic noise [16,35]. Other studies have selected different sites (e.g., streets) within exisiting infrastructures with varying levels of traffic volume and noise [20,32]. Another study design is the provision of different types of nest boxes for cavity breeders that expose the birds to different levels of noise and light, either by their design (e.g., brightened nest boxes with windows [66]) or their position [36,37].

The spatial scale of experimental design studies has usually been small, as nest boxes have been installed and monitored locally or at selected sites across different habitat types. Although some projects monitor nests over longer time periods, the experiments in such settings have only been run over a period of few years. Long-term effects have not been studied. The personnel carrying out the data collection must be trained to carry out bird counts, monitor nests, ring chicks, or maintain technological equipment. Time effort is high as maintenance and data collection are usually daily and span the entire breeding season. The monetary expenditure depends on the material and technical equipment chosen, for example, whether or not speakers over long distances at several sites are installed [16], additional artificial light sources are used, or special nest boxes need to be built [66].

4.3. Bird-Diversity Survey

Detailed data on bird diversity, richness, and abundance in cities have usually been collected by conducting line transects across urban gradients and along different types of traffic infrastructures [23,60,61,68], or point counts within different urban environments, such as residential areas or green spaces [11,16,31,68,69,70] combined with pollution and/or habitat measurements. These methods would provide the basis for detailed investigations of the impact on bird occurrence of environmental factors, such as noise and light pollution, traffic volume, potential disturbance by pedestrians, and also the amount and structure of vegetation. Lines and points must be selected prior to a survey, covering a representative area that includes all study-related stressors with a varying degree. The area covered is highly dependent on the availability of trained personnel to conduct the bird counts and is limited to usually one city or even smaller scales, for example, urban parks. Bird surveys have usually been conducted during spring and in the morning when song activity is highest for most species breeding in a study area. Visits have ranged from one to five within one season, the latter being preferred if study conditions allow. Some studies have repeated the surveys over several breeding seasons or have included autumn and winter bird counts to account for effects on birds all year round or to include habitat use of and impacts on wintering and migrating birds (see e.g., [68]). The personnel carrying out the surveys need to be trained in recognising species by sight and song. Despite this, the monetary costs are relatively low, when not considering the special equipment to record noise, and measure light and air pollution. In some cities, noise and light pollution data are available as they have been measured within citizen-science programmes with smartphone apps [72,73].

4.4. Standardized National Monitoring Programme

National large-scale and long-term bird monitoring schemes by trained birdwatchers have been run for long time series for up to 56 years; thus, they can provide data on species diversity, population trends, distribution patterns, and breeding bird density. They can be a powerful tool to analyse the suitability of different urban environments as breeding habitats for birds. Monitoring schemes have been synchronized and conducted according to standard protocols within a country, and have included the count of abundant (and rare) breeding and wintering birds on a national scale in 28 European countries (Pan-European Common Bird Monitoring Scheme [71]) and North America (North American Breeding Bird Survey [14], and thus provide a strong measure for population trends and bird diversity for countries, or when combined with other national schemes, whole continents. They allow the identifiedof drivers of decreases in species groups, bird communities, functional guilds and overall biodiversity, when combined with remote sensing and other geographical/land-use data. Costs are low due to the involvement of mainly voluntary working field ornithologists, but the organisational effort is potentially, the highest among the different study designs presented here.

4.5. Citizen-Science Projects

Traffic-related stressors, such as noise and light pollution, have been quantified within citizen-science projects that have allowed the coverage of entire cities, countries, and continents. These data are a powerful tool in combination with bird monitoring data (either from standardized monitoring (see above) or citizen-science based), having the potential to study the effects of stressors on bird diversity, distribution, and population trends at large scales.

In the city of Barcelona (Spain), noise maps were created in a pilot study within the larger project ACTION (participatory science toolkit against pollution [73]). A similar project was run in Boston (Massachusetts, USA), providing valuable noise maps of the city and the sources of noise, which were car and lorry traffic on highways and roads [74]. Artificial light pollution has been measured since 2006 by citizen-scientist within the international GLOBE at Night citizen-science campaign (https://www.globeatnight.org/, accessed on 22 November 2022). It was developed by the National Optical Astronomy Observatory (NOAO) and data sampling works via an app (with a personal smartphone) and/or photometers [72,75]. The value of this type of data collection for biodiversity conservation was shown in a study conducted in different towns and villages in Germany. The study compared different street lighting types and their impacts on insect abundance and diversity to ultimately provide useful recommendations for insect-friendly street lighting, while respecting the safety and function for human mobility [47].

Regarding the data sources for bird distribution and diversity, two big databases are used worldwide: (1) The platform eBird (Cornell Lab), on which birdwatchers and amateur ornithologists contribute to a worldwide data collection of bird observations, and its equivalent in several countries in Europe and (2) Ornitho, which has been operating since 2003, and is, for example, in Germany collecting bird observations from ca. 33,000 participants all year round [15]. More specific programmes on urban birds are the Cornell Lab programme Celebrate Urban Birds [76], which was primarily aiming at raising interest for science and nature conservation among all ethnic and social groups in North America [77]. A direct use of research has been the monitoring of nest success by NestWatch, which contributed to a continent-wide study on the effects of noise and light pollution on reproductive success, and supported the previous finding that noise was among the strongest (negative) stressors for urban birds. They found that artificial light affected the timing of breeding events, and potentially led to phenological mismatches, although, in some cases, had beneficial effects on breeding success [30]. Furthermore, national citizen-science projects have targeted birds in proximity to human settlements, i.e., gardens, for example, Christmas Bird Counts (since 1901, USA, National Audubon Society), Garden BirdWatch (since 1995, UK, BTO), and Hour of Garden/Winter Birds (Stunde der Gartenvögel/Wintervögel, since 2006, Germany, NABU), and have conducted bird counts over a long time period on a country scale. Similar to these programmes, but worldwide, the Cornell Lab used the database eBird to collect bird counts within the Great Backyard Bird Count, although the latter suffered from low participation in the past and the potential for bird abundance and diversity assessments was considered limited [78]. Nevertheless, participation in ornithological citizen-science projects has been growing in the last two decades, increasing the data collection of, for example, Great Backyard Bird Count by an order of magnitude [79]. Within the programme ProjektRoadkill (roadkill.at [80], Austria), and also through other national citizen-science and institution-based internet platforms, for example, Tierfund-Kataster (tierfund-kataster.de [81], Germany) (for a global overview see [82]), data on the frequency and location of animals killed by traffic have been collected continuously, and thus, they have provided a database for monitoring the mortality effects of traffic on wildlife in real time.

Data from citizen-science programmes may be used in future studies to specifically address the effects of traffic infrastructures on birds in cities, as most of the data have been collected in residential areas. However, interpretations of results from citizen-science bird counts are limited to studying multi- or single-species population trends, distribution patterns, and overall bird diversity measures, and might not be suitable for the investigation of the mechanisms behind the observed patterns as in experimental designs and/or the use of focal species. Despite these limitations and the inaccuracy of citizen-science-generated data, large-scale drivers of urban bird diversity can be identified by combining citizen-science bird observation data with remote sensing data [2], if the data collected by citizen-scientist offers massive sample sizes and a large geographical and temporal coverage, which would never be achieved under standard monitoring programmes [15].

5. Implications for a Bird- and Human-Friendly Urban Traffic Infrastructure

Green spaces in cities are important for humans (more than in non-urban areas) [83] and for birds, but the type of green space makes a difference in the degree of the beneficial effect [84]. The right selection of greenery along roads could increase bird diversity and nesting opportunities, provide foraging habitat [85], and function as corridors between habitat patches [23,27]. The same beneficial effects of greenery along roads beyond street trees have been found for humans [85], for example, a survey conducted in the centres of two German cities showed high approval of “wilder” and diverse vegetation structures, including shrubs, grasses, and flower beds [86].

The detrimental effects of urban noise and air pollution on human health are numerous [87,88], whereas for birds, the most frequently reported effect of traffic infrastructure and threat in urban environments is noise (see above). In the following, we propose specific suggestions for, or examples of, a “bird-friendly” traffic infrastructure. (1) A significant reduction in speed limits in areas that are identified as valuable for biodiversity would decrease noise levels, as speed has been directly and positively related to noise and collision risk [31,59], and therefore, would decrease the number of road kills and reduce habitat degradation and fragmentation effects at the same time [89]. (2) Wide and dense vegetation belts with many branches, rich understory, and a lot of foliage would function as a noise and air pollution shield [31]. (3) Plants with higher filtering function (e.g., trees with broad leaves) should be intermixed with evergreen species to enable removal of pollutants and also function as shields in winter [90]. (4) Planting or sustaining vegetation of different layers (understory, shrubs, trees) would provide nesting opportunities for different nesting guilds [23]. Nest sites are especially supported by already existing old street trees which should be maintained. (5) Using native species that flower and seed at different times of the year, with local plant diversity, could be enhanced and provide, at the same time, food resources for invertebrates and birds across seasons. Furthermore, the same structures would decrease habitat fragmentation, as wooded/vegetated streets function as corridors for birds [23], bats [91], and other animals [63]. (6) How traffic infrastructures could be illuminated in a “bio-friendly” way and reduce the light pollution of the surroundings has been shown in a study that investigated the effects of different street lights on local insect abundance and richness [47]. Protecting the local insect community would also provide better resource conditions for birds. Furthermore, in experimental settings, birds that were exposed to more light during night chose to actively roost further away from artificial light sources [40], and the occupancy rates for nest sites (in nest-boxes) further away from light sources were higher, clearly showing a selection for darker sites [44]. This suggests that turning off street lighting periodically, choosing the right street light type, and providing structures that provide shade from ALAN, such as vegetated belts or even single shrubs and trees, would benefit urban birds.

Regarding studies in the literature that have investigated the effects of disturbance by pedestrian traffic rate on birds, we conclude that these are negligible. Structures that increase safety and wellbeing for humans, for example, the construction of bike lanes separate from high-traffic roads [64], ultimately, would be beneficial for bird diversity through lower noise levels, lower air pollution, and more vegetation. Moreover, in this review, we have highlighted that, in urban environments, the co-use of green traffic infrastructure is key when aiming for improvements in both human wellbeing and bird diversity. Whereas birds do not directly benefit from the presence of humans, they do indirectly benefit when structures are designed that are more positively perceived by humans, as the same structures improve habitat quality for birds (in terms of nesting and foraging sites), improve connectivity between populations, and reduce traffic-related disturbances.

6. Discussion

Empirical studies have mostly investigated the indirect effects of different traffic infrastructures, i.e., airports, highways, cross-country train tracks, major and minor roads, and the disturbance effects by pedestrians, on bird diversity, population trends of birds, and their reproductive success. Most of these studies have been conducted in rural environments. In cities, detailed investigations of these effects are scarce and limited to comparisons of different road types and their connectivity to urban green spaces, but the impacts of, for example, different road types, bike lanes, tram and within-city train tracks on urban bird community and performance have not been investigated and should be the subject of future studies. Furthermore, studies have mostly been conducted on a small scale, usually within one city. There has been no study at a broader scale, such as at a country level, or a comparison between different cities globally, that may give a more general picture of the effects of traffic infrastructure on bird diversity in cities, lifting the topic above their geographical and structural context. In general, traffic has been found to have negative impacts on birds, but also on human wellbeing. Among the stressors reviewed here, we consider noise and light pollution to be the biggest threats for urban birds, so that other traffic-load related factors could be neglected, if these two factors are present or addressed, respectively. So far, numerous studies have shown that human wellbeing and bird diversity are linked [5,6,70,92,93]. Thus, we conclude that to make urban environments liveable and sustainable for both humans and birds, traffic infrastructure needs to be designed in a way that traffic-related stressors are reduced, most importantly noise [87,94], and so that diverse vegetation structures are available, to improve mental health and human wellbeing [86,95] and also offer diverse nesting and foraging sites for different bird species [23,27]. Traffic infrastructures should be suitable to connect higher quality habitats by functioning as corridors between urban greenspaces [63]. The Urban greenspaces exhibit the highest biodiversity in cities [70,84], and by facilitating the movement of wildlife between them, urban biodiversity would be improved. The latter, and the access to recreational areas, are key to human wellbeing [6]. Following the “one health” theory [96], the quality of life for citizens and birds can and should be improved simultaneously [92,97,98]. Therefore, we state that to create urban habitats for both humans and wildlife, habitat quality must be assessed not only by looking at the benefits of human mobility when designing traffic infrastructure, but also regarding the suitability for sustaining healthy populations of humans and wildlife, for example, by designing “biophilic streets” [85]. This includes the provision of features for human recreation and safe mobility, and also animal movement corridors between suitable habitat patches, nesting opportunities, and foraging grounds for animals.