Urbanization, characterized by the rapid expansion of human settlements and infrastructure, has become a global phenomenon, transforming natural environments with significant implications for biodiversity (Grimm et al., 2008; Mckinney, 2008). By 2010, over 50% of the world’s population resided in urban areas, and projections indicate that by 2050, this figure will rise to 70% (UN, 2012). For Africa, the projected tripling of urban land cover by 2030, is particularly concerning as it will lead to substantial habitat loss across the continent (Seto et al., 2012). This ongoing urban development will result in increased predator presence (Baker et al., 2008; Fischer et al., 2012), elevated noise levels (Proppe et al., 2013), and forest fragmentation (Zipperer et al., 2012), with substantial influence on community composition (Marzluff, 2001), and ecosystem functioning (e.g. Blair, 1996; Ferenc et al., 2014; Lim and Sodhi, 2004; Marzluff, 2001; Pautasso et al., 2011).

The composition of bird communities in urban areas is a function of species tolerance and adaptive capacity. Species sensitive to habitat disturbances are classified as “urban avoiders” (McKinney, 2002) or “urban-sensitive” (Garden et al., 2007), while those thriving in urban environments are called “urban exploiters” (McKinney, 2002) or “synanthropes” (Marzluff et al., 2001; Cresswell et al., 2020). Both categories are impacted differently by urbanization (DeGraaf and Wentworth, 1986; Isaksson, 2018), with specialized species (because of their life-history and ecological traits) being impacted the most (Evans et al., 2011; Sol et al., 2014). The negative effects of urbanization on bird communities have been extensively documented globally and in Africa (see Chamberlain et al., 2017; Oliviera et al., 2017; Ibáñez-Álamo et al., 2017; Lee et al., 2021; Sol et al., 2020). Considering the growing human population and the subsequent increased urbanization globally (United Nations Department of Economic and Social Affairs, 2019), understanding species’ responses to anthropogenic pressures are paramount for guiding conservation measures (Newbold et al., 2018).

Because birds are well known, and are good indicators of environmental health (Fraixedas et al., 2020), they are invaluable when studying the impacts of urbanization on habitat structure and composition (Chace and Walsh, 2006; MacGregor-Fors et al., 2009; Reis et al., 2012; Bregman et al., 2014; Marzluff, 2016). Numerous studies have documented varying trends in how urbanization affects bird abundance and diversity (Palacio et al., 2018; Carvajal-Castro et al., 2019; Korányi et al., 2021). While some studies report minimal influence of urbanization on bird diversity (Korányi et al., 2021), others link increased urbanization to reduced diversity (Sol et al., 2017, 2020). Conversely, a few studies report increased bird diversity with rising urbanization (Batáry et al., 2018; Filloy et al., 2019). Beyond bird abundance and diversity, it is essential to consider birds for the ecological roles they play in seed dispersal, pollination, pest control, nutrient cycling, and scavenging (Lundberg and Moberg, 2003; Pigot et al., 2016; S̜ekercioğlu et al., 2016; Sekercioglu, 2006). These roles not only sustain ecosystems but also ensure human wellbeing (Cardinale et al., 2012); for example, loss of scavengers, has been shown to lead to an increase in diseases (García-Jiménez et al., 2021, 2022).

The functional traits that support bird’s roles in environments have been extensively documented in recent studies (Pigot et al., 2020; Tobias and Pigot, 2019), allowing for detailed characterization of bird communities with unmatched precision. To better understand impacts of urbanization on vital ecosystem functions and services (Sol et al., 2020), an effective method is to examine functional diversity. Functional diversity encompasses the identity, variety, and relative abundance of phenotypic traits in organisms that affect key ecosystem processes (Díaz et al., 2007; Petchey and Gaston, 2006; Tilman, 2001). Studies are particularly needed in the tropical regions including Africa, where rapid population growth has led to accelerated urbanization (Seto et al., 2011, 2012). However, only a few studies have investigated how urbanization affects bird ecological roles in Africa (Chamberlain et al., 2017; Lee et al., 2021; Njoroge et al., 2014; van Rensburg et al., 2009; Awoyemi et al., 2024). Awoyemi et al. (2024) examined the impact of urbanization on biodiversity metrics within cities in a single country, utilizing a paired sampling approach to compare urban and non-urban areas. To gain a more detailed insights, future studies could explore the effects of urbanization on bird community and ecosystem functions across a finer urbanization gradient and at a broader scale.

Here, we investigate the impacts of urbanization and human modification on taxonomic and functional metrics both in Nigeria and Kenya. Nigeria and Kenya average population densities are estimated at 226 humans/km² and 97 humans/km², respectively (United Nations, 2019). Both countries fall within the Afrotropical region with rich but declining biodiversity (Cazzolla Gatti et al., 2015). Nigeria is situated on the East Atlantic Flyway, while Kenya sits on the East Asia-East Africa Flyway, making both countries crucial for the conservation of migratory birds. Yet, the impact of urbanization on biodiversity in these landscapes is relatively understudied (Ibáñez-Álamo et al., 2017; Magle et al., 2012), mainly due to limited local capacity and funds (Awoyemi and Ibáñez-Álamo, 2023).

Building on Oliviera et al. (2017), Newbold et al. (2015), and Sol et al. (2017), we compare bird diversity across four levels of urbanization (settlement types): cities, suburban areas (moderately urbanized environments), dispersed rural areas (slightly human-modified), and mostly uninhabited areas (MUAs) in Kenya and Nigeria. We hypothesize that urbanization negatively impacts taxonomic and functional richness and diversity of African bird communities. Specifically, as urbanization intensifies, we expect to observe a decline in both the number of species (taxonomic richness) and the unique ecological function of the species (functional richness). Additionally, we anticipate that urban expansion will reduce the overall diversity of bird communities, leading to more homogenized assemblages with fewer distinct species and functional roles. We also anticipate that uninhabited and rural areas will have higher taxonomic and functional richness and diversity than suburban areas and cities, reflecting the negative impact of urbanization on bird communities.

A total of 914 species across 89 bird families were recorded in Nigeria, while 1069 species across 94 families were recorded in Kenya. In both countries, the majority of species (>90%) are classified as Least Concern, with 27 Threatened species recorded in Nigeria (3 Critically Endangered, 11 Endangered, and 13 Vulnerable) and 44 in Kenya (5 Critically Endangered, 20 Endangered, and 19 Vulnerable) according to the IUCN Red List (BirdLife International, 2024). Kenya supports 225 habitat specialists, compared to 179 in Nigeria. Additionally, both countries host urban-adaptive species, with 40 in Nigeria and 50 in Kenya. A detailed list of species is provided in Supplementary Data Sheet S2 .

Urbanization, as a form of global change, has been shown to significantly alter the physical environment (Kaye et al., 2006), creating new environments and opportunities for species. Consequently, urbanization often leads to variations in bird community metrics across different gradients (Sol et al., 2020; Petersen et al., 2022). Our results indicate that taxonomic richness and diversity were notably higher in Mostly Uninhabited Areas (MUAs) compared to cities. MUAs, characterized by more natural or “pristine” vegetation, offer a variety of habitats conducive to diverse bird communities. The availability of suitable habitats is critical for species distribution and with urbanization natural or near-natural environments are transformed for housing, industrial, or commercial purposes, leading to habitat loss for bird communities (Donnelly and Marzluff, 2006; Sandström et al., 2006; Croci et al., 2008). This change shape communities by filtering out some species while allowing others to replace them. In some cases, certain bird species may remain if fragments of near-natural habitats persist (Haire et al., 2000).

Over time, urbanization results in the replacement of specialist species by generalists (Oliviera et al., 2017; MacLean et al., 2018), as specialists are more susceptible to environmental changes. As the specialists are filtered out of the urban areas (Evans et al., 2011; Sol et al., 2014; Fischer et al., 2015), the unique functional traits they contribute to ecosystem functioning are lost, potentially leading to lower functional richness in the urban centers with crucial conservation implications (Fischer et al., 2015; Sol et al., 2014). This corroborates findings from other studies indicating that urbanization drives species decline (McKinney, 2006; Ibáñez-Álamo et al., 2017). On the other hand, functional diversity increased with the urbanization gradient, with cities exhibiting a higher functional diversity than MUAs. However, a continuous increase in HMI revealed a possible decline, indicating that while cities may initially support a diverse range of functional traits, excessive human modification eventually reduces this diversity after reaching a tipping point. This pattern suggests a transitional phase in bird communities, particularly in newer cities, where species are still adapting. Over time, as seen in established cities like Nairobi, only a few adaptable species may persist, leading to shifts in community composition, such as the disappearance of Hooded Vultures (Necrosyrtes monachus), which were once common in Kenya (Odino et al., 2014), and the proliferation of Marabou Storks (Leptoptilos crumeniferus). This “undulating” relationship might reflect the dynamic nature of species adaptation in response to urbanization. Additionally, while functional diversity is expected to decline at some point with urbanization (Fischer et al., 2015; Sol et al., 2020), our findings agree with Lee et al. (2021), who reported slightly higher functional diversity in urbanized areas as compared to non-urbanized areas. One explanation for this could be because functional diversity is an abundance-weighted metric that is easily influenced by the proliferation of generalist and disturbance tolerant species. Urbanization could also result in humans providing other sources of food through human modification of habitats. For example, the gardens, refuse dumps and ornamental plants could provide additional sources of food in urban areas (Pauw and Louw, 2012).

Our findings reveal variability in the relationship between the urbanization measures and bird community metrics, thus highlighting the importance of employing various metrics in evaluating the effects of urbanization on bird communities (Matuoka et al., 2020; Sol et al., 2020; Lee et al., 2021; Petersen et al., 2022; Suárez-Castro et al., 2022). These findings are critical for managing landscapes especially in the face of growing urbanization. HMI negatively influenced taxonomic diversity, taxonomic richness and functional richness but had an unstable relationship with functional diversity. Functional diversity initially declined with the human modification index, then followed by a subsequent increase in the functional diversity as the human modification increased. Our result suggests that though species richness declines, there may be no clear change in abundance along the urbanization gradient and in some cases certain species may inflate the total abundance of individuals (Chamberlain et al., 2017). One possible reason for this is that the total abundance may limit species richness such that where there are more individuals, the resources available may not be able to support more species, thus some species will have small populations, leading to their extirpation in such environments (Gaston, 2000). For this reason, species richness could be lower in cities (with higher human modification index), while functional diversity is either maintained (Lee et al., 2021; Callaghan et al., 2023) or higher as we have found. As more habitats are modified with increasing urbanization, there will be fewer habitats available to meet the niche requirement of bird communities (Grimm et al., 2008; Pautasso et al., 2011; Oliviera et al., 2017). For example, there is a decline in food availability along an urbanization gradient (Shochat et al., 2006), which thus limits the species found within the cities. However, cities may provide more feeding opportunities for raptors as well as reduced predation on them (Chace and Walsh, 2006). Thus, settlement-types with some natural habitats will provide more habitats to meet the natural requirements of more bird species, thus leading to an increase in taxonomic and functional richness.

Both the Human Modification Index (HMI) and the likelihood of urban expansion negatively influenced taxonomic diversity, taxonomic richness, and functional richness but positively influenced functional diversity. Our results suggest that although species richness declines, the overall abundance of individuals may not change clearly along the urbanization gradient. In some cases, certain species may disproportionately contribute to the total abundance (Chamberlain et al., 2017). One possible reason for this is that total abundance may limit species richness such that in environments with more individuals, available resources may not support additional species, leading to smaller populations and eventual extinction of some species (Gaston, 2000). Consequently, species richness could be lower in cities (with higher Human Modification Index), while functional diversity is either maintained (Lee et al., 2021; Callaghan et al., 2023) or higher, as we have found. As more habitats are modified with increasing urbanization, there will be fewer habitats available to meet the niche requirements of bird communities. For example, food availability declines along an urbanization gradient (Shochat et al., 2006), limiting the species found within cities.

Habitats are lost due to urbanization and specialists are replaced by generalists (De Coster et al., 2015), resulting in high species turnover rates. This could result in some niches not being utilized, thus providing opportunities for invasive species to explore (Jonason et al., 2017). Such opportunities could also lead to high populations of specific species, which could increase functional diversity in cities. For example, some studies have shown urbanized areas to have more individuals of fewer species (Chace and Walsh, 2006; Chamberlain et al., 2017). This is a probable reason why species richness declined while functional diversity increased with the urbanization gradient.

Furthermore, functional richness is determined by the presence or absence of specific traits in the niche space, while species-abundance-based diversity and differences among species influence functional diversity (RaoQ index) (Botta Dukát, 2005). This further highlights the effect that the number of individuals has on functional diversity. As a result, functional diversity may decrease if species richness increases. As species with unique traits are filtered out, it increases the average dissimilarity among species. On the other hand, both intermediary levels of urbanization and the MUAs had higher species richness due to the availability of suitable habitat, which allows the occurrence of both specialists and generalists. Additionally, if species occupying the same functional space replace each other, it does not influence functional trait diversity, thus masking species loss and changes in species composition when only functional diversity is considered (De Coster et al., 2015). For example, habitat loss had little or no influence on functional diversity but resulted in species loss and changes in species composition in forest bird communities (Coetzee and Chown, 2016; Matuoka et al., 2020). This highlights the need to incorporate more metrics in understanding the influence of urbanization on bird communities since functional diversity alone may not provide a comprehensive understanding of changes in bird communities.

The continuous growth in population will cause more people to live in cities by 2050. This highlights the need for a functioning ecosystem as it is critical for human well-being (Cardinale et al., 2012). While our results add to the already existing body of knowledge on the effects of urbanization on bird communities, it also gives insight into the future. Our results provide evidence that urbanized areas will allow functional diversity to increase while functional richness, taxonomic richness and diversity decline. A hundred percent probability of urban expansion leads to decrease in taxonomic richness, functional richness and taxonomic diversity but at the same time results in a corresponding increase in functional diversity. This result is critical for landscape management and urban planning. We should be cautious to draw conclusions that functional diversity increasing with urbanization is positive as this may not necessarily benefit bird conservation, ecosystem functioning and human wellbeing. As generalists replace specialists, they are able to provide a wide range of functional traits within urban centers. With more generalists, there is high niche overlap which eventually causes functional redundancy. While increased functional diversity in urban environments may mitigate the impact of species loss on ecosystem function to some extent, the loss of specific species within the ecosystem results in the disappearance of their unique traits. This can create gaps within the ecosystem, particularly, in communities with lower functional redundancy (Winfree et al., 2015). The replacement of specialists by generalists can also lead to functional redundancy as some niches are left unoccupied. For instance, if insectivorous birds that control insect population (Whelan et al., 2015) are filtered out of the cities, it could lead to pest outbreaks. Also, an increase in green areas will result in a corresponding increase in habitat diversity within cities, thus creating more niches that meet the ecological needs of bird communities (Oliviera et al., 2017). We therefore suggest that urban planners should incorporate natural vegetation within urban centers to maintain other community and functional indices within urban centers.