1 Introduction

As global urbanization accelerates, changes in urban environments pose a significant threat to biodiversity. However, urbanization also provides a unique opportunity to study how organisms adapt and evolve in new environments (Isaksson, 2018). Birds, as a well-studied taxonomic group, exhibit diverse and complex adaptive responses to urban settings. The study by Mascarenhas et al. (2022) indicates that environmental changes driven by urbanization may promote local adaptation in birds, particularly in behavioral and physiological traits. Understanding how birds adapt to urban environments not only contributes to biodiversity conservation but also provides scientific insights for urban ecosystem management and policy development (Alexandrino et al., 2019).

Urbanization represents an extreme transformation of natural habitats into artificial environments, leading to severe habitat fragmentation and biodiversity loss. During the process of urbanization, many bird populations decline, and some species even disappear. However, certain species manage to survive and even thrive in urban settings (Martin and Bonier, 2018). Selective pressures in urban environments, such as air pollution, artificial lighting, noise, and human disturbances, drive significant phenotypic changes in some bird species (Caizergues et al., 2022).

This study aims to analyze existing literature to explore the impact of urbanization on birds’ adaptive traits and survival, identifying which traits contribute to their persistence in urban environments and how these changes reflect evolutionary processes. By uncovering the effects of urbanization on bird population dynamics and ecological niches, this study seeks to provide new perspectives and insights for future urban ecology research and biodiversity conservation strategies.

2 Urbanization as a Driver of Environmental Change

2.1 Key characteristics of urban environments

Urban areas are characterized by impervious surfaces, high human density, and pollution, which collectively create a distinct ecological setting (Leveau and Ibáñez, 2022). These environments often favor bird species with specific traits, such as nesting in buildings and having uniform plumage, which help them exploit urban resources and avoid predation. Urban birds tend to be generalists with large niche breadths, allowing them to adapt to the diverse and fluctuating resources available in cities (Callaghan et al., 2019).

2.2 Differences between urban, suburban, and rural ecosystems

The ecological dynamics of urban, suburban, and rural ecosystems differ significantly. Urban areas typically support higher bird abundance due to the dominance of synanthropic species, while suburban areas may offer a balance of natural and anthropogenic resources, leading to peak survival rates for some species (Kurucz et al., 2021). In contrast, rural areas often provide more stable and less disturbed habitats, supporting different species compositions and ecological interactions.

2.3 How urbanization influences resource availability and bird habitats

Urbanization influences resource availability by introducing novel food sources and altering habitat structures. While urban environments can provide abundant anthropogenic resources, such as food waste, they also pose challenges like increased predation risks and habitat fragmentation (Evans et al., 2015). Birds in urban areas often exhibit behavioral and physiological adaptations, such as increased boldness and altered stress responses, to cope with these changes (Caizergues et al., 2022). However, the availability of specific resources, such as nesting sites and food, can vary widely across urban gradients, affecting bird community composition and survival (Sepp et al., 2018).

3 Behavioral Adaptations of Birds to Urbanization

3.1 Changes in foraging behavior and diet flexibility

Urban environments often favor bird species with broad dietary preferences and flexible foraging behaviors. Generalist species, capable of exploiting diverse food resources—including anthropogenic foods—are more likely to thrive in cities, while specialists tend to decline. Urban birds frequently shift their diets to include seeds, fruits, and human-provided foods, and omnivory is a common trait among successful urban dwellers (Jokimäki et al., 2016; Callaghan et al., 2019; Neate‐Clegg et al., 2023).

3.2 Modification of nesting sites and breeding strategies

Birds in urban areas adapt their nesting behaviors by utilizing artificial structures such as buildings and cavities, which can offer protection from predators and environmental extremes (Jokimäki et al., 2016; Callaghan et al., 2019). Ground-nesting species, however, are particularly vulnerable and often decline in urban settings due to increased disturbance and habitat loss (Lakatos et al., 2022). Some urban-adapted species also exhibit changes in breeding strategies, such as prolonged breeding seasons and increased clutch sizes, potentially compensating for lower reproductive success (Patankar et al., 2021).

3.3 Altered migration patterns and site fidelity

Urbanization acts as a filter on migratory behaviors, with resident and short-distance migrant species more likely to persist in cities than long-distance migrants. Urban environments may disrupt traditional migration routes and timing, leading to altered site fidelity and, in some cases, reduced migratory behavior (Lakatos et al., 2022; Neate‐Clegg et al., 2023).

3.4 Responses to human presence and urban threats

Urban birds often display increased boldness and reduced fear of humans, facilitating their survival in densely populated areas (Patankar et al., 2021; Caizergues et al., 2022; Neate‐Clegg et al., 2023). Behavioral shifts such as higher exploratory behavior and aggressiveness have been observed, though these changes are not always adaptive and may sometimes be maladaptive (Caizergues et al., 2022). Additionally, urban birds must contend with unique threats, including noise, pollution, and increased predation risk, which further shape their behavioral responses (Isaksson, 2018; Patankar et al., 2021; Neate‐Clegg et al., 2023).

4 Morphological and Physiological Adaptations

4.1 Body size and plumage variations in urban bird populations

Urban environments often select for birds with smaller body sizes and, in some cases, longer tails, as these traits may confer advantages in maneuvering through fragmented habitats and coping with urban challenges (Santos et al., 2024; Neate‐Clegg et al., 2023). However, the effects of urbanization on body mass and size are highly species-specific, with some species showing reduced body size while others remain unaffected (Brouwer et al., 2022; Neate‐Clegg et al., 2023). Plumage variations, though less frequently documented, may also arise due to altered predation pressures and environmental conditions in cities (Isaksson, 2018).

4.2 Changes in metabolism and energy expenditure

Urban birds may experience changes in metabolism and energy expenditure as a result of altered food availability, increased stress, and exposure to pollutants. Genetic studies have identified selection on genes related to energetic metabolism in urban populations, suggesting physiological adaptation to the energetic demands of city life (Mascarenhas et al., 2022). Additionally, shifts in hormonal state and body condition have been observed, reflecting physiological responses to urban stressors (Patankar et al., 2021; Reid et al., 2024).

4.3 Adaptation to urban microclimates and artificial structures

Cities create unique microclimates characterized by higher temperatures, artificial light, and increased pollution. Birds that thrive in urban areas often exhibit physiological and morphological flexibility, enabling them to exploit artificial structures for nesting and to tolerate urban microclimates (Jokimäki et al., 2016; Isaksson, 2018). The ability to use buildings and other man-made features as nesting sites is a key trait among successful urban species (Jokimäki et al., 2016; Neate‐Clegg et al., 2023). These adaptations, combined with physiological resilience, are critical for survival in the face of rapid urban expansion.

5 Vocal and Communication Adjustments

5.1 Changes in song frequency and structure due to noise pollution

Birds in urban areas often increase the frequency and amplitude of their songs to overcome low-frequency noise from traffic and human activity. These modifications help their calls remain audible in noisy environments but can also alter the structure and complexity of their songs (Patankar et al., 2021; Pena et al., 2023). Such changes are widespread among urban-dwelling species and are considered a key adaptive trait for urban survival (Patankar et al., 2021).

5.2 Effects on bird communication and mating success

Altered vocalizations can impact communication efficiency and mating success. While higher song frequencies may improve signal transmission in noisy environments, they can also reduce the information content or attractiveness of songs, potentially affecting mate choice and reproductive success (Patankar et al., 2021). The long-term consequences of these changes on population dynamics and fitness remain an important area for further research (Patankar et al., 2021).

5.3 Case studies of urban songbird adaptation

Numerous studies document urban songbird species, such as great tits and other passerines, that have successfully adjusted their vocalizations to urban noise. These species demonstrate increased song frequency and amplitude compared to their rural counterparts, highlighting the role of vocal plasticity in urban adaptation (Patankar et al., 2021; Pena et al., 2023). However, the degree and effectiveness of these adaptations can vary by species and city, reflecting the complexity of urban selective pressures (Patankar et al., 2021).

6 Genetic and Evolutionary Responses to Urbanization

6.1 Evidence of rapid evolution in urban bird populations

Recent studies demonstrate that urban bird populations can undergo rapid evolutionary changes in response to city-specific challenges. For example, research on a Neotropical bird species (Coereba flaveola) identified genetic signatures of local adaptation in urban populations, with selection acting on genes related to metabolism, gene regulation, immune function, and nervous system development (Mascarenhas et al., 2022). These findings suggest that urban environments can drive swift evolutionary responses, often within a few generations (Isaksson, 2018; Patankar et al., 2021).

6.2 Genetic markers associated with urban adaptation

Genome-wide scans have revealed specific genetic markers and loci under selection in urban birds. In Coereba flaveola, 46 loci were identified as putative selection outliers, with 30 linked to biological processes relevant to urban adaptation, such as energetic metabolism and immune response. Additionally, genes involved in nervous system development showed signatures of selection, indicating a genetic basis for observed behavioral adaptations in urban settings (Mascarenhas et al., 2022).

6.3 Natural selection pressures and microevolution in urban birds

Urban environments impose unique selection pressures, including altered food resources, increased pollution, noise, and novel predators. These pressures can lead to microevolutionary changes, such as shifts in behavior, physiology, and morphology (Isaksson, 2018; Patankar et al., 2021). For instance, urban birds often display increased boldness, altered stress responses, and changes in hormonal state, which may be underpinned by genetic differentiation from rural populations (Patankar et al., 2021). However, not all phenotypic shifts are necessarily adaptive; some may result from non-adaptive processes or population filtering rather than direct selection (Isaksson, 2018; Caizergues et al., 2022).

7 Challenges to Bird Survival in Urban Areas

7.1 Increased predation risk and competition with invasive species

Urbanization often alters predator-prey dynamics and increases competition with invasive species. Ground-nesting and ground-foraging birds are particularly vulnerable in cities, as urbanization favors predators and invasive species that thrive in disturbed habitats, leading to declines in sensitive native bird populations (Jokimäki et al., 2016; Lakatos et al., 2022). Urban areas also tend to support a higher abundance of generalist and synanthropic species, which can outcompete specialists and further reduce biodiversity (Jokimäki et al., 2016; Callaghan et al., 2019; Neate‐Clegg et al., 2023).

7.2 Effects of pollution (air, water, and light) on health and reproduction

Birds in urban areas are exposed to multiple pollutants, including air, water, and light pollution, which can negatively affect their health and reproductive success (Isaksson, 2018; Reid et al., 2024). Air pollution and artificial light at night are linked to physiological stress, altered hormonal states, and reduced immune function, while water pollution can impact food availability and chick development (Isaksson, 2018; Reid et al., 2024). The effects of pollution are often species- and context-dependent, with some health metrics and life stages (such as juveniles) being more negatively affected than others (Reid et al., 2024).

7.3 The impact of climate change and extreme weather events in urban settings

Urban areas can exacerbate the effects of climate change, such as increased temperatures and more frequent extreme weather events, due to the urban heat island effect and reduced natural habitats (Isaksson, 2018). These changes can further stress bird populations, particularly those with narrow habitat requirements or limited dispersal ability. The combination of urbanization and climate change may lead to shifts in community structure, reduced diversity, and increased vulnerability of certain species (Isaksson, 2018; Neate‐Clegg et al., 2023).

8 Case Analysis: The Urban Success of the European Blackbird (Turdus merula)

8.1 Historical background and expansion into cities

The European Blackbird was historically a forest-dwelling species but has become one of the most common birds in European towns and cities. Its successful urban colonization is linked to behavioral flexibility, such as nesting in buildings and trees, and a diverse, omnivorous diet that allows it to exploit a wide range of urban food resources (Jokimäki et al., 2016; Kurucz et al., 2021). The blackbird’s ability to breed in artificial structures and adapt its feeding habits has facilitated its widespread presence in urban centers (Jokimäki et al., 2016).

8.2 Behavioral and physiological adaptations to urban environments

Urban blackbirds exhibit notable behavioral changes, including increased boldness and reduced fear of humans, which are advantageous in city settings. They also show shifts in life history traits, such as prolonged breeding seasons and increased clutch sizes, likely as a response to altered predation pressures and food availability in urban areas (Patankar et al., 2021). Physiologically, urban blackbirds may display changes in hormonal state and body condition, reflecting adaptation to urban stressors like noise, pollution, and artificial light (Figure 1) (Patankar et al., 2021; Isaksson, 2018).

8.3 Genetic divergence between urban and rural populations

Urban and rural populations of blackbirds can show genetic differentiation, with urban birds often exhibiting distinct genetic markers associated with behavioral and physiological traits that enhance urban survival (Patankar et al., 2021; Isaksson, 2018). These genetic differences may arise from both natural selection and population filtering, as individuals with traits suited to urban environments are more likely to thrive and reproduce in cities (Patankar et al., 2021).

8.4 Conservation lessons from the european blackbird’s urban adaptation

The urban success of the European Blackbird highlights the importance of behavioral flexibility, generalist diets, and the ability to exploit artificial structures for urban colonization (Jokimäki et al., 2016; Patankar et al., 2021). Conservation strategies for urban birds should focus on maintaining habitat diversity and supporting species with a range of adaptive traits. However, the blackbird’s success also underscores that urban environments tend to favor generalist species, potentially leading to reduced overall avian diversity and the decline of specialists (Jokimäki et al., 2016; Patankar et al., 2021; Isaksson, 2018).

9 Conservation Strategies for Urban Bird Populations

9.1 Urban Planning and Bird-Friendly Green Spaces

The design and maintenance of diverse, connected green spaces are critical for supporting urban bird diversity. Parks with high habitat connectivity and those located farther from city centers are more effective at conserving species with high habitat specificity (Tai et al., 2021). Incorporating a variety of vegetation types, including trees and shrubs, and preserving or restoring grasslands can help sustain both generalist and specialist species (Lakatos et al., 2022; Santos et al., 2024). Urban planners should prioritize the protection and enhancement of these habitats to maintain avian biodiversity (Callaghan et al., 2019; Tai et al., 2021; Santos et al., 2024).

9.2 Mitigating negative impacts of urbanization

Reducing light and noise pollution is essential for improving bird health and reproductive success, as these stressors are linked to physiological stress and altered behaviors (Patankar et al., 2021; Reid et al., 2024). Maintaining or increasing green cover, especially with native plant species, can buffer the negative effects of urbanization by providing food, shelter, and nesting sites (Pena et al., 2023; Santos et al., 2024). Additionally, managing invasive species and minimizing habitat fragmentation are important for reducing competition and predation pressures on native birds (Jokimäki et al., 2016; Lakatos et al., 2022).

9.3 Public engagement and citizen science initiatives for bird conservation

Citizen science projects and public education campaigns play a vital role in urban bird conservation. Large-scale citizen science efforts have provided valuable data on bird survival, abundance, and responses to urbanization, informing conservation management and policy decisions (Brouwer et al., 2022). Engaging the public in monitoring and habitat restoration fosters a sense of stewardship and increases awareness of urban biodiversity challenges (Jokimäki et al., 2016; Brouwer et al., 2022). These initiatives can also help identify local conservation priorities and track the effectiveness of implemented strategies.

Acknowledgments

The authors extend sincere thanks to two anonymous peer reviewers for their feedback on the manuscript.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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