Our study further confirms that functional traits are powerful tools for understanding how species respond to environmental factors, including urban stressors (Petchey & Gaston 2002). Our results support the hypothesis that, although birds and bats are both flying vertebrates, their functional responses to urbanisation differ, while some strategies remain conserved within urban heat islands in both groups.
4.1. Different functional responses of urban bird and bat assemblages to strong artificialisation
Although highly urbanised sites, such as UHIs, exhibited higher Mean Functional Distance (MFD) in bird assemblages, this was not accompanied by increased Functional Richness (FRic). This incongruence is not surprising. Abundance-weighted metrics such as MFD are not linearly related to species richness, while trait-space volume metrics like FRic almost always are (de Bello et al. 2016). These peculiarities help us understand why functional diversity appeared higher in highly urbanised sites, as UHIs, than in UCIs. Such patterns may be linked to the composition of urban bird assemblages. In our case, Poitiers’ urban bird community is dominated mainly by Passeriformes, leading to low evolutionary distinctiveness (Querejeta et al. 2025). In fact, closely related communities have shown to have high trait divergence in several cases, potentially due to ecological opportunity and rapid trait divergence (Losos 2008; Oliveira Hagen et al. 2017). This divergence is, indeed, reflected in the functional space, through dbRDA ordination, where UHIs showed a broader ellipse than UCIs, highlighting greater variation in trait composition across sites. Our results echo findings from other studies (Lee et al. 2021; Oliveira Hagen et al. 2017). For instance, Oliveira-Hagen et al. (2017) reported higher avian functional diversity in urban areas than in nearby rural sites, even after controlling for species richness. They attributed this “paradox” to the habitat heterogeneity offered by urban environments compared to more homogeneous rural landscapes. A similar process seems to operate in our study area: UHIs are more heterogeneous than UCIs. This heterogeneity likely contributes to the higher functional diversity we observed. However, this may be only part of they as our non-significant fourth-corner analysis shows that bird traits’ distribution is not entirely shaped by urban landscape variables. This suggests that other processes may also be at play. One possibility is the role of distinctive or functionally rare bird species, which appear to increase along the urbanisation gradient. These species may be a key driver of functional diversity in birds, unlike in bats, where such a pattern was not detected.
Concerning bird functional rarity, we can confirm our initial hypothesis as our results indicate that the disappearance of distinctive species would strongly affect functional diversity. This pattern is consistent with previous work showing that rare bird species often contribute disproportionately to the functional structure of assemblages (Leitão et al. 2016). Their loss could, thus, have cascaded consequences and ultimately compromising the long-term provision of ecosystem services. In our study, the distinctive species grey heron (Ardea cinerea) illustrates this point well. As the sole representative of the order Pelecaniformes and one of the few aquatic predators specialised in wetland habitats, it maintains a high share of functional diversity despite its rarity. By contrast, bat assemblages do not exhibit the same pattern as the disappearance of functionally distinctive species appears to have little effect on overall functional diversity. In contrast, functional diversity appears to be shaped mostly by the collective contribution of all species, or by the dominance of abundant generalists such as the common pipistrelle, as we had initially hypothesized. This interpretation is supported by our fourth-corner analysis, which shows that bat functional assemblages in urban areas are strongly structured by landscape variables. Taken together, these results suggest that bats are directly affected by anthropogenic stressors and may, therefore, be more vulnerable to intense urbanisation than birds. Indeed, bird functional diversity is strongly influenced by rare species, which serves as a reminder that their disappearance could have disproportionate impacts on entire assemblages.
Bird and bat Functional Redundancy (FRed) increased in highly urbanised sites (UHIs) compared to UCIs. This finding aligns with the idea that urbanisation reduces trait diversity and promotes functional homogenisation, whereby several species fulfil similar functional roles (Palacio et al. 2018). In our study, this is illustrated by the dominance of generalist traits, such as granivores, which tend to replace more specialised traits, such as insectivores, thereby increasing FRed within UHIs. Generalists can be considered redundant due to their high niche overlap (Palacio et al. 2018). While FRed may buffer ecosystems against species loss in the short term, it can also reduce ecosystem resilience in the long term by diminishing functional complementarity. Indeed, some urban bird assemblages have already lost nectivorous pollinators, indicating low FRed for that particular ecological role (Pauw & Louw 2012). However, urban areas will never host as many species as natural habitats due to intense human pressures such as buildings, roads, and constant human activity. Moreover, the limited number of microhabitats available within cities constrains the range of ecological niches. Therefore, maintaining both high FRed and substantial MFD in urban bird communities is essential for promoting ecosystem resilience. FRed ensures that key ecological roles are preserved even if some species decline, whereas a high MFD enables communities to retain a broad array of ecological strategies, allowing them to cope with environmental changes and to exploit the full range of available urban habitats. Indeed, in our bird assemblages, we observed that high functional diversity, as measured by MFD, co-occurs with substantial FRed. Indeed, it has been argued that FRed and diversity are not mutually exclusive, as species may be functionally similar in some effect traits while remaining distinct in response traits, thereby maintaining both ecological insurance and niche differentiation (Fischer & de Bello 2023). Similarly, communities may be able to retain high functional diversity together with redundancy across environmental gradients, suggesting that environmental heterogeneity promotes multiple coexisting strategies that enhance ecosystem resilience (Monge-González et al. 2021). Indeed, in our study, UHIs exhibited higher landscape heterogeneity. Overall, our study reveals a complex pattern of both congruence and divergence in bird and bat functional diversity metrics along the urbanisation gradient.
4.2. Urban landscape variables influence bat but not bird functional traits
Our study found no significant associations between urban landscape variables and bird functional traits based on the fourth-corner analysis. This suggests that other factors, such as the presence and influence of functionally rare species, may be driving the observed patterns in urban functional structure. This finding also aligns with a potential environmental filtering process that has already shaped bird colonization before reaching the urban core (Querejeta et al. 2025; Sol et al. 2014). In other words, the effect of landscape variables likely occurs along the rural-to-urban gradient, rather than between UCIs and UHIs. In contrast, bat functional traits showed significant associations with urban landscape variables, indicating an ongoing process of functional filtering along the urbanisation gradient, from UCIs to UHIs driven by the higher sensitivity to urban stressors. Among the associations detected, bat species were found to rely more on human-made structures and less on underground locations for roosting as the distance from natural and semi-natural areas increased. This pattern supports the idea that species with greater flexibility in roost-site selection are more successful colonisers of urban environments (Jung & Threlfall 2018), highlighting the importance of maintaining and restoring natural roosting and maternity sites to enhance bat species and functional diversity in cities.
The strongest positive association between landscape and functional traits was observed in bat species that primarily feed on Lepidoptera, which were positively correlated with the density of vegetation cover. Indeed, urban moth communities are highly sensitive to vegetation structure, making this finding ecologically meaningful (Tyler 2020hätalo et al. 2024). In our study, key Lepidoptera-feeding bats, including species from the genera Rhinolophus, Plecotus, Barbastella, and Myotis, were predominantly found in UCIs rather than in UHIs. This suggests that the availability of Lepidoptera, and feeding resources in general, may act as a limiting factor for these species in urban settings. Accordingly, we recommend the implementation of urban green infrastructure that prioritizes native plant species, high-quality natural habitats, and ecological connectivity with surrounding rural and natural areas to promote ecological continuity.
4.3. Certain birds and bats functional traits are conserved within UHIs
Overall, our study has revealed that bird and bat colonisation of highly dense anthropised sites is represented by favouring certain traits over others within UHIs. In the case of birds, Community Weighted Means of Hand-Wing Index or HWI, geographical distribution range, lifespan, Community Specialization Index or CSI and Community Temperature Index or CTI are explained by the PC1, as they all increase exponentially along a gradient with higher values of urbanisation. Birds with higher HWI, indicating more pointed wings and greater dispersal ability, were more conserved in UHIs. This aligns with the idea that strong dispersers tend to have larger geographic ranges and are better equipped to colonise novel or disturbed environments, such as urban areas (Arango et al. 2022; Claramunt et al. 2022; Neate-Clegg et al. 2023). In addition to HWI, species with broad distribution ranges appear to also thrive in the highly urbanised landscape, suggesting that dispersal ability plays a key role in successful urban colonisation. Indeed, bird species found in European urban areas have been shown to exhibit greater dispersal abilities compared to those in more natural habitats (Møller 2009). Thus, dispersal abilities would enable birds, located in UHI to cover large areas and therefore seek resources in sites that are less hostile or competitive than UHI. An alternative explanation is that the observed variation in HWI may arise from differences in foraging modes, including flycatching and aerial insectivory, rather than from dispersal ability per se (Neate-Clegg et al. 2023). Moreover, the increase in lifespan associated with artificialisation may be related to a decrease in predation pressure at highly artificial sites (Eötvös et al. 2018), where birds may have chosen a strategy of living longer in order to learn to exploit urban environments (Neate-Clegg et al. 2023). Urban areas are warmer than their surroundings because built surfaces absorb and release heat (Aram et al. 2019). This effect peaks in dense zones such as UHIs. Accordingly, the rise in CTI with urbanisation reflects increasing thermal stress, with thermophilic species dominating the most modified environments (Barnagaud et al. 2012; Piano et al. 2017). At the same time, CWM of tarsus length is higher in UCIs, when comparing to UHIs, and decreases with higher values of PC2. Indeed, birds’ tarsus lengths are associated with locomotory performance, escape flight ability and, thus, capacity to avoid predators (Amiot et al. 2022). One hypothesis would lead us to expect a higher number of predators in less urbanised areas and, thus, more predation pressure (Eötvös et al. 2018). Therefore, this take-off capacity would lead birds to higher changes of survival within UCIs. However, the smaller tarsus lengths observed in urban passerines, the so-called “urban winners”, have been linked to the production of lower-quality offspring. This is concerning, as in species such as sparrows, the final tarsus length is typically attained at fledging (Gosler et al. 1998). This supports a second hypothesis, that increased urbanisation may reduce offspring quality in highly anthropised sites, potentially triggering a long-term degradation of urban bird communities and increasing the risk of local extinctions. Nonetheless, further observational and experimental research is needed to determine which mechanisms are driving these patterns.
In relation to bat morphological traits, CWMs of forearm length and body mass were higher within UHIs and their variation was explained by the type of urban point, UHI and UCI. Longer forearm lengths are known to be closely associated to faster flight in open areas (Wood & Cousins 2023) and, potentially, more dispersal capacity, capacity which was also conserved in more urbanised sites in the case of birds. Indeed, a recent study has shown that certain UK urban bat assemblages were larger than in rural areas, potentially due to the absence of pesticides increasing insect availability (Hughes et al. 2024). In line with this, higher body mass may also lead to faster flight speed and larger flights which would make foraging activities more efficient (Jung & Threlfall 2018), and potentially in areas further from urban stressors. In fact, some light-tolerant bat species, such as the common pipistrelle (Pipistrellus pipistrellus) and the common noctule (Nyctalus noctula), are known to forage under city lights while maintaining their roosting sites in rural areas outside towns (Hale et al. 2012; Mathews et al. 2015). At the same time, echolocation is a defining trait in bats, shaping how they perceive and interact with their surroundings (Denzinger & Schnitzler 2013). Our results highlight call peak frequency and duration as key traits influencing adaptation to highly urbanised sites. Longer calls and lower peak frequencies increased with artificialisation, consistent with studies showing that such signals perform better in noisy, open urban environments (Avila-Flores & Fenton 2005; Jung & Threlfall 2018; Wolf et al. 2022). Lower frequencies reduce masking by high-frequency noise, while longer calls enhance spatial resolution (Bunkley et al. 2015; Hage et al. 2013). Conversely, according to fourth-corner results, call durations shortened with denser vegetation, supporting evidence that acoustic clutter promotes shorter, broadband calls (Suarez-Rubio et al. 2018; Starik & Göttert 2022). Undoubtedly, in strongly urbanised areas such as UHIs, morphological, echolocation, ecological, and thermal traits that increase the fitness and survival of birds and bats are consistently favoured. Yet, whether these traits are also different in surrounding rural areas requires further exploration of rural, peri-urban and urban assemblages. In the case of bats, shifts of activity, especially related to hunting behaviour would help shedding light onto the distribution of functional traits due to human modification.
Overall, we have found not only dissimilarities but also similarities on the functional traits, which confer tolerance and/or vulnerability in birds and bats to urban environments. This leads us to reject our hypothesis in which functional traits that are associated with the colonisation of UHIs and UCIs were different between birds and bats. While the effect on some functional traits is specific from birds, such as more longevity and bigger distribution ranges in strong urbanised sites, and to bats, such as echolocation calls characteristics, several functional characteristics follow the same patterns in both groups. Indeed, higher dispersal capacities in highly dense urbanised sites related with bigger wings, as measured by HWI in birds and forearm lengths in bats is a common advantageous trait to colonize highly urbanised sites. It is, hence, plausible that only bird and bat species with high dispersal capacity can reach and persist in anthropised environments, migrating from rural roosts into cities where competition may be lower due to environmental filtering. However, longer wings because of longer forearms may be associated with a different diet more than to a real adaptation to urban environments. Moreover, higher specialisation and higher thermal tolerance associated with UHIs is another common characteristic related to habitat requirements. In fact, the higher specialization may be associated with higher landscape heterogeneity found in UHIs in comparison to UCIs. Despite species-specific traits, our analyses reveal that community composition along the urbanisation gradient is driven by the filtering of conserved functional traits that shape bird and bat distributions across UHIs and UCIs.
4.4.Towards multifunctional urban landscapes for conservation
Conservation assessments based solely on assemblage composition or taxonomic diversity risk missing key ecological processes and can therefore lead to suboptimal conservation and urban management decisions (Laureto et al. 2015). Indeed, our functional approach has revealed key findings that may help enhance biodiversity in urban areas. For instance, our results suggest that bird assemblages are partly structured by functionally rare species, and that their local disappearance would disproportionately erode functional diversity. This implies that conservation efforts for urban birds should explicitly prioritise these functionally rare ‘outliers’ (Violle et al. 2017). In our case, this is mainly represented by aquatic predators such as the grey heron, whose distinctive traits and low redundancy make them irreplaceable contributors to ecosystem functioning. Aquatic avian predators are widely known to be scarce in urban environments. Similar patterns have been reported for fish-eating waterbirds in the highly urbanised New York/New Jersey Harbour, where suitable wetland habitats are limited. In that case, a dedicated conservation programme protects nesting islands and associated wetlands, couples habitat restoration with disturbance management, and relies on long-term monitoring to safeguard these top predators within a densely urbanised landscape (Craig et al. 2015). Ultimately, urban conservation strategies should aim to maintain the ecological functions carried by these trait outliers, notably through wetland protection, improved river habitat connectivity, and targeted management of vulnerable aquatic predator populations. At the same time, our study has shown low abundance of insectivorous birds compared to granivorous within the study area. Indeed, previous studies have shown that enhancing tree diversity and vegetation complexity in urban green spaces can increase insectivory by birds (Schillé et al. 2025). This measure could also be beneficial to increase the diversity of Lepidoptera-feeding bats in UHIs as insectivorous bats are known to respond positively to structurally complex vegetation and abundant insects near water bodies (Suarez-Rubio et al. 2018; Straka et al. 2020). Moreover, promoting high-quality insect communities within urban green-blue infrastructures has been identified as a key measure for conserving insectivorous predators (Mata et al., 2017). Together, these patterns highlight that increasing native vegetation diversity, reducing light and chemical pollution, restoring ecological continuity, and improving wetland and riparian habitats are essential steps to sustain insectivorous birds and bats. Finally, integrating functional traits into the design and management of multifunctional urban spaces could help support low-dispersal bird and bat species in UHIs and other highly urbanised areas. By increasing the local availability of food and nesting resources in these dense urban patches, species with limited dispersal capacity may be able to colonise and persist in sites that are currently beyond their reach. Studies of multifunctional urban green–blue infrastructure demonstrate how explicit design, implementation and management of multiple ecosystem services can support urban biodiversity (Cook et al., 2024; Vierikko et al. 2014). For example, in the GREEN SURGE Urban Learning Labs, pilot green–blue projects combining vegetation and water features provided habitat together with social and climatic benefits, confirming that biodiversity-friendly multifunctionality is achievable. Such conservation solutions need inter- and transdisciplinary research that effectively connects ecological knowledge with urban planning and decision-making.