Discussion

49 Regarding the subgroup analysis, we found an average of 21.5 (SD ± 2.1) species in the reference sites and an average of 16.4 (SD ± 2.4) species in the urban subgroups. Our hierarchical clustering results for incidence and abundance values showed a clear separation of the reference sites from all the urban subgroups (Fig. 4).

Figure 4. Hierarchical cluster dendrograms depicting avian community composition patterns in the subgroups analyzed for a) incidence data (Sørensen index) and b) abundance data (Bray- Curtis index).

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to record all five seasonality categories of birds known for Xalapa (i.e., residents = 59%, winter residents = 23%, migrants = 7%=, seasonal visitors = 4%, summer residents =1%, and the remaining 6% are species with two different seasonal statuses in Xalapa), a fact that is more likely linked to the location of the city along one of the most important migratory routes for Nearctic-Neotropical migrants (Ruelas Inzunza et al., 2005). This fact highlights the importance to carry out our surveys coinciding with the arrival of the earliest summer residents, seasonal visitors, transient/migratory, and winter residents before they return to their breeding grounds, having a more complete representation of the avian communities on the region.

We found that our hypothesis of a spatial association of urban communities linked to non-urban reference sites was not supported by our results, at least considering the time period in which we performed our study. A similar result was found in Mexico City, where the composition was not influenced by the adjacent environments (Puga-Caballero et al., 2014). Also, in boreal countries (i.e., France, Finland, and Canada) has been reported that urban bird communities are independent of the avian communities in the adjacent landscapes, being most important the environmental variables as occurs in our study (Clergeau et al., 2001). Instead, following our alternative hypothesis, we found statistically significant relationships between vegetation cover percentage (measured remotely) and the similarity values of the bird communities analyzed. This result is similar to what has been reported in the literature showing the importance of intrinsic environmental variables shaping the structure of avian communities (Donnelly and Marzluff, 2006; Escobar-Ibáñez et al., 2020;

Ortega-Álvarez and MacGregor-Fors, 2009; Villaseñor et al., 2020). This phenomenon is particularly evident in the city of Xalapa, where vegetated areas cover an estimated of ~40%

of its territory, being considered a green city (Falfán et al., 2018). The importance of the resources that vegetation supply for birds and other wildlife in cities are not only evident in the spatial composition of their communities but also in the functional identity of the species that are part of them, showing differences in the guilds present in each area (Ortega-Álvarez et al., 2018).

Regarding the results of our hierarchical cluster dendrograms, despite the assessed compositional similarity obtained with both incidence (Sørensen) and abundance-based (Bray-Curtis) indices, we found a similar clustering in both of them, with reference sites

51 being separated from the urban groups (Fig. 4). This result was not surprising considering the marked differences between both groups (i.e., reference points vs. citywide survey points). This is because the characteristics of both reference sites contrast with the citywide survey points. In the case of the non-urban reference points, human activities are not so extreme (mainly limited to jogging, walking, and birdwatching, among some others with a relatively low disturbance level), in relation with the sites located within the urban matrix more exposed to anthropogenic disturbance (e.g., vehicle and pedestrian activities, traffic noise, urban infrastructure at variable intensities, air and water pollution). However, urban avian communities are usually affected differentially according to with certain traits such as, specialization, diet, breeding and nesting habitat, migratory status, and social behavior to name a few (Croci et al., 2008; Devictor et al., 2007; Evans et al., 2018). Mostly, urbanization acts as a multidimensional filter, involving intrinsic (e.g., traits and mechanisms) and extrinsic (e.g., infrastructure, food predictability, human activities, urban hazards) factors affecting birds at the individual level and varying spatially and temporally (MacGregor-Fors et al., 2022).

Previous studies have indicated that invertivores (generally referred to as insectivores in most studies), as the most benefited feeding niche represented in Neotropical avian communities both in natural and urban areas (Barbosa et al., 2020; Campos-Silva and Piratelli, 2021; Escobar-Ibáñez et al., 2020; Ortega-Álvarez and MacGregor-Fors, 2011). Our results for both the city-wide survey and both non-urban reference sites concur with this pattern (52% of species are invertivores). Interestingly, this result contrast with other studies in which invertivore or insectivore species are mostly associated with forested areas and more sensitive to the environmental changes (Canaday, 1996; Lim and Sodhi, 2004). However, caution must be taken as it appears that this pattern is regionally dependent of local habit characteristics playing an important role in urban communities (e.g., tropical versus temperate regions and total availability of greenspaces within the urban area).

Five of the six species shared among the three analyzed sites (urban, South, and North) are among the most abundant breeding (Brown Jay [Psilorhinus morio], Common Chlorospingus and Great-tailed Grackle) and wintering species (Black-throated Green Warbler and Wilson's Warbler) in the urban area of Xalapa. All of them are associated with open and semi-open anthropogenic ecosystems (Escobar-Ibáñez et al., 2020; MacGregor-

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Fors et al., 2018). However, we also recorded 21 unique species in the reference sites not found within the urban area during our surveys. Brown-backed Solitaire, Gray-breasted Wood-Wren (Henicorhina leucophrys), Chestnut-capped Brushfinch (Arremon brunneinucha), and Golden-browed Warbler (Basileuterus belli) have been reported as resident species that thrive in well-preserved forest understory (Ortega-Álvarez et al., 2018).

These and other forest birds usually depend on shrubby understory vegetation to forage, nest, and get cover from predators (Campos-Silva and Piratelli, 2021; Macchi et al., 2019). The shrubby forest understory is reduced or even absent in some parts of the city and this kind of landscape is very contrasting in relation with the habitats found in natural environments (Hedblom and Söderström, 2010). Urban vegetation is usually unequally distributed and its mainly tailored to fit citizen’s necessities and aesthetic values, compromising their ecological roles (Falfán and MacGregor-Fors, 2016). Having said that, not all species respond equally to urban vegetation management. Contrastingly, some migratory species have been associated with highly developed areas, some examples being the Orange-crowned Warbler (Leiothlypis celata), Wilson's Warbler, and Lincoln's Sparrow (Melospiza lincolnii; Ortega- Álvarez et al., 2018), all of them recorded in our citywide survey.

Within the city, we recorded other species known as important urban exploiters such as the invasive species House Sparrow (Passer domesticus), Great-tailed Grackle, and Rock Pigeon (Columba livia), which have been considered as some of the most common species in urban areas. In this study, we also recorded some species belonging to the taxonomic families considered the most commonly found in cities around the world such as doves (Columbidae), tyrant flycatchers (Tyrannidae), and woodpeckers (Picidae; Aronson et al., 2014; Lepczyk et al., 2017). Some studies have indicated that developed areas can be unsuitable for cavity nesters (e.g., Picidae) due to the absence of dead trees (Souza et al., 2019), however, we recorded three species of this family and even the Golden-fronted Woodpecker (Melanerpes aurifrons), which is a very common species in many parks and gardens of Xalapa (González-García et al., 2014).

4.1 Conclusions

Our results showed that urban avian communities do not always follow a structure similar to those in adjacent non-urban areas. In the case of Xalapa, there is no pattern of association

53 between the communities outside and within the city. The urban avian communities are rather influenced by the available resources supplied by urban parks and particular gardens located throughout the city. These human related resources potentially benefit the species with more tolerance and that are capable of going through the ‘filter’ that urbanization imposes on the most specialist species (MacGregor-Fors et al., 2022). Also, vegetation patches (e.g., parks, gardens, vacant lots, vegetated sidewalks) within the urban areas could be used as green corridors by individuals adapted to tolerate modified areas and the changing conditions inherent to urbanization to enter and persist in the cities. We were able to determine that the highest similarity values followed the highest percentage of vegetation, remarking the importance of this resource to allow urban birds to tolerate or even thrive in urban environments. In terms of conservation, this could be very relevant if the objective is to make our cities more wildlife friendly. It could also be used as a parameter to understand the necessities and requirements not only to have a richer avifauna but also to ensure their persistence and safety in our cities.