Santiago’s native and exotic species pool
The high proportion of native species in Santiago (89%) agrees with the high native/exotic ratio reported in other cities in the world [4, 14, 15]. However, the total species richness (46 species) is below the median of bird species in other cities of the Neotropics (above 100 species) . There are urban gradient study cases which suggest that other large cities in the Neotropics share richness values comparable to Santiago, such as Mexico City  and Guadalajara  in Mexico, with 51 and 58 species respectively. In Argentina, Leveau & Leveau  reported 31 bird species in the city of Mar del Plata. Other urban gradient analyses in Southern Chile suggest that bird richness values in the cities of Valdivia, Temuco and Osorno ranges between 30 to 50 species [67, 68]. However, these cities are smaller than Santiago. Cities located in other biogeographic realms in the Southern Hemisphere also possesses higher species richness than Santiago. Particularly the Afrotropic and Indo-Malayan realms, where cities display a median richness above 200 species. Santiago’s bird species richness is similar to the median number of species reported for cities in the Australasian realm (Southern Hemisphere), and in the Nearctic realm (North Hemisphere) . It is important to emphasize that the high native/exotic species ratio in Santiago contrasts with the observed pattern for plant species in this city, where 85% of the flora is exotic . In this context, it has been proposed that urban areas with more native plant species tend to retain more native bird species , because the native plant species proportion  and volume of native vegetation  is positively correlated with native bird diversity in urban settlements. A note of caution must be considered here, because it has been stablished that the explanation for the uncommonly high proportion of exotic bird species (44%) in Dunedin, New Zealand, is largely independent of plant distribution . Nevertheless, we suggest an investigation of the relationship between the proportion of exotic plants and the relatively low bird species richness of Santiago.
It is noticeable that, despite Santiago being located within the “Central Chile EBA”, none of the 13 endemic birds of Chile  are represented among Santiago’s urban dwellers. The absence of endemic birds in this city occurs because just three of the 13 endemic birds of Chile birds share distribution ranges that include the city of Santiago. These are cursorial species, with very specific habitat requirements for nesting and cursorial displacement, which are probably not satisfied inside urban centers (the Moustached Turca (Pteroptochos megapodius), the White-throated Tapaculo (Scelorchilus albicollis), and the Chilean Tinamou -Nothoprocta perdicaria-) [55, 72]. Nevertheless, there are numerous eBird registers of these species around Santiago’s urban perimeter. Therefore, we consider that the accelerated urbanization in the Central Chile EBA could be a threat to these endemic species.
Santiago’s exotic bird species fraction is within the range of the low proportion of exotic bird species reported for cities in the Neotropics and other biogeographic realms . It appears that all exotic species with compatible habitat requirements and enough propagule pressure around Santiago have already become successful urban dwellers. All this despite exotic species being a reduced fraction of this urban avifauna. The exotic bird species in this city (five species) belongs to the omnivorous and granivorous guilds. Two exotic birds share cosmopolitan distribution and their native range is located in Europe and Asia (The omnivorous Columba livia and Passer domesticus); the granivorous California quail (Callipepla californica) has its native range in North America. On the opposite, other exotics come from native distribution ranges located in South America (The omnivorous Molothrus bonariensis and the granivorous Myiopsitta monachus) . Although they represents only 11% of the Santiago’s urban bird assemblage, they comprise more than one half of the total exotic bird species pool (nine species) reported for the country . Among Chile’s invasive birds not present in Santiago (four species), there are two aquatic species whose habitats requirements are not satisfied in the city (The Muscovy Duck, Cairina moschata) and the Cattle Egret, Bubulcus ibis). The other two invasive species not considered as urban dwellers in Santiago are the Ring-necked Pheasant (Phasianus colchicus) and the Red-crested Cardinal (Paroaria coronata) . Even though the latter species display omnivorous trophic habits (one of the richest trophic guilds in Santiago), they are not represented in this city. This could be explained by the fact that, despite considered invasive, their populations in Chile are scarce and they possibly do not attain enough propagule pressure to colonize large cities.
The distribution of the functional groups at Santiago
The migratory species fraction in Santiago is relatively low compared to the complete urban avifauna. This pattern raises the necessity to stablish the extension of the relevant regional bird species pool for Santiago, in order to test if urban/regional migratory species ratio is in agreement with the idea that urbanization selects against migrant bird species [20, 22, 73]. Nevertheless, an analysis about the fraction of migrant and resident bird species in different urban systems must be encouraged for conservation issues, because it has been established that migrants experience higher conservation risks than resident birds in urban environments . In addition, there are examples of cities with a high proportion of bird species in some migration category, such as the South American city of La Paz, Bolivia . In a similar way, we have established that the proportion of confirmed urban nesting species in Santiago is high (67%), even though the proportion of species with uncertain (7%) or no information (11%) about urban nesting is considerable. Either way, more complete and detailed information about Santiago’s nesting patterns is strongly needed in order to asses if some nesting strategy is favored in this city, as cavity-nesters or above-ground nesters do in other cities in the world [20, 76].
The highest species richness among Santiago’s trophic guilds occurs in the insectivorous, followed by omnivorous, granivorous, and carnivorous. By considering the number of registered individuals as a proxy of abundance relationships, it is possible to hypothesize that Santiago’s avifauna is numerically dominated by omnivorous, followed by granivorous and insectivorous in third place. The spatial distribution of the interpolated species richness and number of individuals, suggest that species of the previously mentioned guilds are able to exploit a larger extent of urban habitats than the carnivorous guild, which display large areas where both the interpolated species richness and number of individuals tends to zero. They also reach a maximum number of individuals, which is several magnitude orders higher than those observed for the carnivorous guild. This pattern agrees with evidence from cities in several countries across the world (e.g. Finland, Mexico, Venezuela, Canada and France). which suggest that highly urbanized habitats can select for omnivorous species [12, 15, 21, 25, 73], which can exploit an additional food supply associated to human activities, especially in cities where winter imposes a reduced food supply and a restrictive energetic budget for birds. Singapore is a city located in a tropical area, where winter food supplies and energetic budget is non-restrictive. Contrary to the former examples, Singapore’s highly urbanized areas can select for granivorous, while omnivorous are not favored in the extremes of the urban gradient . The same pattern of selection for granivorous in highly urbanized habitats has been reported in a study conducted in three different ecoregions in the United States . Santiago is in a Mediterranean - type area, where the rainy season falls in winter, with moderate temperatures. This climatic setup is comparable to that of the city of Jerusalem, where omnivorous have been found to be the dominant trophic guild in the downtown area .
Insectivorous birds are considered to be negatively affected by the degree of urbanization, and there are numerous examples of low representation and selection against species of this guild in several cities [3, 12, 15, 20,21,22]. Nevertheless, there is growing evidence of high insectivorous richness coming from cities located in the Neotropics [19, 23, 25, 75, 77] and Indo-Malayan realms [14, 78]. In sum, this evidence suggests that urban environments outside the Palearctic and Nearctic realms can sustain a high insectivorous richness. Santiago’s distribution of species richness and number of individuals shows that insectivorous birds are able to exploit habitats across most of the urban surface, being the guild with the highest species richness, and the third most abundant among Santiago’s trophic guilds. The mechanisms that potentially explain the high insectivorous richness and abundance should be investigated in order to provide more evidence about the importance of the insectivorous guild in cities of the Neotropical realm. In this context, it has been proposed that insectivorous birds in Mexico City are the dominant group, but they are highly represented only in parks, large gardens, and other areas where trees and vegetation are abundant . Supporting this argument, de Toledo et al.  propose that urban environments can, in fact, provide a large food supply for insectivorous birds, because arthropods are often a diverse and abundant group in urban environments. Therefore, the effects of vegetated areas and degree of prey availability on the distribution of insectivorous birds should be evaluated for Santiago.
Altogether, carnivorous distribution of interpolated species richness, and absolute number of individuals, suggest that this guild is the least successful (among the four proposed multispecific guilds) in exploiting the urban environment of Santiago. The carnivorous guild appears to be restricted to a few spots were species richness is relatively high, but abundance seems to be low, compared to omnivorous, granivorous and insectivorous. The fact that carnivorous are represented by several species in Santiago, despite being geographically and numerically restricted, is in agreement with the explanation proposed by Chace & Walsh , which stablished that carnivorous as raptors may possess home ranges larger than city boundaries, not requiring to satisfy all their ecological needs inside the city. Several carnivorous species registered in Santiago are raptors, whose nesting pattern and trophic habits inside the city are unknown. Therefore, their observed urban distribution could be a consequence of a broad home range, which allows them satisfy -at least partially- their ecological needs outside the boundaries of Santiago.
Overall, we hypothesize that food availability for birds in the urban habitats of Santiago allow the city to sustain a high richness and abundance of omnivorous (which can benefit from anthropogenic food), granivorous and insectivorous (which are likely to find their alimentary items across the parks and open spaces within Santiago). Other guilds display trophic requirements that are hypothetically more restrictive in Santiago, such as nectarivorous, frugivorous, herbivorous and piscivorous. These guilds are mono-specific and reach lower numbers, and show a reduced and patchy geographic coverage (Additional file 3), as compared to the multi-specific guilds which numerically dominate the city. Considering the complete avifauna, the spatial location of high richness and abundance spots is distributed across Santiago’s urban surface. No obvious diversity gradient appears, despite the fact that registers are denser in the central and eastern parts of the city, situated between the main rives of Santiago (Fig. 1).
In consequence, what challenges are emerging from this promising system for the urban avifauna? Twenty two percent of Santiago’s bird species have an unknown or uncertain nesting status; this lack of information is critical, because seven of the 10 species classified into the “probable” or “without information” nesting categories belong to the insectivorous and carnivorous guilds, which we propose as interesting traits of Santiago’s avifauna. Therefore, more detailed information regarding nesting categories for urban birds, and descriptions about urban nesting patterns, should be encouraged at this city. For the same reasons, dietary studies inside the urban perimeter of Santiago must be considered a priority, because current trophic categorization of Santiago’s birds comes from evidence that has not been evaluated inside urban habitats.
Finally, the use of the citizen based platform eBird has revealed the coarse structure of Santiago’s functional groups, and suggested the geographic distribution patterns and overall abundance relationships. As a corollary, we sustain that the combined efforts of citizens and researchers has positioned eBird as a useful tool to reveal the general distribution of urban avifauna, and to potentially reveal patterns at a geographical scale which would be otherwise prohibitive, in a traditional sampling approach. This information framework encourages its use to continue enhancing the use of this tool in ornithological research. Simultaneously, we suggest further research projects about the city’s avifauna, in several topics such as the characterization of the habitats experienced by Santiago’s urban birds inside the city, the degree of urbanization, the vegetation coverage, and the food availability for different trophic guilds.